Fix simple solidify wrong custom data on large ngons

Fixes an unreported issue that vertex data on large ngons (>255)
is messed up due to type conversion to char and back to int.

Ref D10734

Modifier: Add "Connected" mode to the weld modifier

Implement improvement from T73139 for merging along edges.
It is now called "Connected" mode, while the default is called "All".

With the recent performance improvement, the Connected Mode is in some
cases only double the speed than the usual merge all strategy but in
other cases it may be even faster. The bottleneck is somewhere further
down the line of merging geometry.

The motivation for this patch came from T80897, because the merging in
complex solidify is making it very slow.
Now merging can be removed from solidify without greater consequences,
as this is just a quicker and more advanced algorithm to do the same
thing that solidify currently does slowly.

Reviewed by: mano-wii, campbellbarton

Ref D8966

Add Custom Object Space to Constraints

Add Custom Space to the list of space conversions for constraints.

Constraints can use World Space, Local Space, Pose Space, Local with
Parent, and now also Custom Space with a custom object to define the
evaluation space.

The Custom Space option uses the Local Space of an other
object/bone/vertex group. If selected on owner or target it will show a
box for object selection. If an armature is selected, then it will also
show a box for bone selection. If a mesh object is selected it will show
the option for using the local space of a vertex group.

Reviewed By: #animation_rigging, sybren, Severin, angavrilov

Differential Revision: https://developer.blender.org/D7437

Add Custom Space to Constraints

Constraints can currently only use World Space, Local Space, Pose Space, Local with Parent.
This patch adds Custom Space with a custom object to define the evaluation space.

The Custom Space option uses the Local Space of an other object/bone/vertex group.
If selected on owner or target it will show a box for object selection.
If an armature is selected, then it will also show a box for bone selection.
If a mesh object is selected it will show the option for using the local space of a vertex group.

It looks like this in the UI:
{F8731291}

The motivation for this patch is the following situation.
If you have a copy location constraint in your armature you are already limited when you want to use a specific axis. It can be done, but its unneccessarily difficult (as far as I know). But the situation is close to impossible as soon as you have a root bone in your armature (which I was told you always want). I need to only copy the location along a custom axis for a lot of things. The same goes for copy rotation/scale and all the limit constraints because I need to make that axis relative to the root bone transform.

Here is a demo video:
{F8498960}
This is the file of that video:
{F8647541}

This is the file for the tests included in this patch:
{F8699677}

File intended for acceptance test:
{F8879818}

Reviewed By: #animation_rigging, sybren, Severin, angavrilov

Differential Revision: https://developer.blender.org/D7437

Transform: Correct Mirror for Object Mode along arbitrary axis

This fixes T68521, T82334.

The object are not properly mirrored in object mode.

For mirroring an object that has an arbitrary rotation along the X axis
this is the procedure:
1. mirror x location.
2. negate x scale of the
3. negate y and z component of the rotation (independent of which rotation mode is used).

This knowledge applies now for all angles and axes to finally make the
mirror operation work in object mode.

The new mirror function has the downside that it can not (in this form)
be used with proportional editing.

This is no problem since the old behavior can still be replicated by
scaling with -1 along any axis.

The solution to get perfect mirrors can not work for scaling in general,
because in that case there could be scew created.

Reviewed By: mano-wii

Differential Revision: https://developer.blender.org/D9625

Add Custom Space to Constraints

Constraints can currently only use World Space, Local Space, Pose Space, Local with Parent.
This patch adds Custom Space with a custom object to define the evaluation space.

The Custom Space option uses the Local Space of an other object/bone/vertex group.
If selected on owner or target it will show a box for object selection.
If an armature is selected, then it will also show a box for bone selection.
If a mesh object is selected it will show the option for using the local space of a vertex group.

It looks like this in the UI:
{F8731291}

The motivation for this patch is the following situation.
If you have a copy location constraint in your armature you are already limited when you want to use a specific axis. It can be done, but its unneccessarily difficult (as far as I know). But the situation is close to impossible as soon as you have a root bone in your armature (which I was told you always want). I need to only copy the location along a custom axis for a lot of things. The same goes for copy rotation/scale and all the limit constraints because I need to make that axis relative to the root bone transform.

Here is a demo video:
{F8498960}
This is the file of that video:
{F8647541}

This is the file for the tests included in this patch:
{F8699677}

File intended for acceptance test:
{F8879818}

Reviewed By: #animation_rigging, sybren, Severin, angavrilov

Differential Revision: https://developer.blender.org/D7437

Fix T80897: Complex solidify crash when special ngons are merged

This has serious performance implications as the additional check makes
it O(N^2) when there are a lot of merged vertices.
The goal is to move the merging to the weld modifier where it would
be much easier to optimize.

Ref D8946

Fix T80893: Complex solidify special case with singularity

Ref D8929

Fix T80895: Complex solidify compare edges by content

Even if the index of the edge is not equal the content can be equal
if it has been merged. So compare the content.

Ref D8931

Modifiers: Use vertex group options for complex solidify flat faces

Ref D8945