Merge branch 'master' into greasepencil-object

tweak: copy transforms

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

Refactor the Custom Space patch.

Instead of modifying all the get_constraint_targets and
flush_constraint_targets callbacks, introduce wrapper API
functions and handle the new reference there.

Also, it is not needed to do all the target dance to compute
the space_obj_world_matrix.

Constraints: add support for a new Owner Local Space for targets.

Add a new transformation space choice for bone constraints, which
represent the local transformation of the bone in the constraint
owner's local space.

The use case for this is transferring the local (i.e. excluding the
effect of parents) transformation of one bone to another one, while
ignoring the difference between their rest pose orientations.

Owner Local Space replaces the following setup:

* A `child` bone of the `target`, rotated the same as `owner` in rest pose.
* A `sibling` bone of the `target`, positioned same as `child` in rest
pose and using Copy Transforms in World Space from `child`.
* The `owner` bone constraint uses Local Space of `sibling`.

Differential Revision: https://developer.blender.org/D9493

Limit Rotation: add an Euler Order option and orthogonalize the matrix.

Since Limit Rotation is based on Euler decomposition, it should allow
specifying the order to use for the same reasons as Copy Rotation does,
namely, if the bone uses Quaternion rotation for its animation channels,
there is no way to choose the order for the constraint.

In addition, add a call to orthogonalize the matrix before processing
for the same reasons as D8915, and an early exit in case no limits are
enabled for a bit of extra efficiency.

Since the constraint goes through Euler decomposition, it would remove
shear even before the change, but the rotation won't make much sense.

Differential Revision: https://developer.blender.org/D9626

Copy Transforms: implement Invert, Fix Shear and more Mix options.

This constraint can be naturally viewed as a prototype for a future
4x4 matrix math node (or subset thereof), since its basic semantics
already is matrix assignment. Thus it makes sense to add math options
to this constraint to increase flexibility in the meantime.

This patch adds support for several operations that would be useful:

- An option to fix shear in the incoming target matrix.
- An option to invert the target matrix.
- More ways to combine target and owner matrix.

Shear is known to cause issues for various mathematical operations,
so an option to remove it at key points is useful. In the future node
system this would be a separate operation, but due to the limits of
the constraint stack it has to be built in for now.

Inverting a matrix is also an operation that can be useful to have.
For some uses it may be useful to invert components separately, so
implement this by checking the Mix mode setting to avoid UI options.

Finally, add two more ways to combine the matrices (multiplied by
two due to the necessity for the Before/After choice). Now there
are three combine modes:

Full implements regular matrix multiplication as the most basic option.

Split Channels combines location, rotation and scale separately.
Looking at D7547 there is demand for such a mode in some cases,
and even with nodes it's cumbersome to rig manually every time.

Finally, Aligned emulates the 'anti-shear' Aligned Inherit Scale mode,
and basically uses Full for location, and Split for rotation/scale.

Differential Revision: https://developer.blender.org/D9469

Fix crash in outliner on cursor hover

The previous commit surfaced a bug introduced in rBab4654cdfe8f in which
a boolean wasn't initialized to false.

Merge branch 'master' into geometry-nodes

Outliner: Highlight icons on cursor hover

The icons for collapsed children already draw highlighted when hovered.
Because the item icons are now select targets (for outliner to properties
editor tab switching) this adds highlights on hover for all outliner
element icons.