Merge branch 'master' into sculpt-mode-features

Mask filter: add mask filter tool with iterations

Now it is possible to test the operator without editing the keymap.
By changing the iterations of the filter it should be possible to avoid
adding an unnecessary amount of undo steps when working with high poly
meshes.

Fix Windows build

Sculpt mode transform tool: Initial implementation

Work in progress...
- Scaling and orientations are not implemented yet.
- The "move pivot only" option is ignored when the tool is used from the
gizmo.
- Only meshes, no multires or dyntopo.

This commit also includes the operator to set the pivot point position
automatically as well as a refactor of the mesh filter cache to be shared
with the transform tool.

Sculpt: Mask by color operator

Similar to mask by normal, this operator generates a mask based on the
color of the active vertex.

Mesh Filter: Multithreaded initialization and masked node filtering

This should improve performance a lot, making the tool usable on several
millions of vertices. If the mesh is partially masked, only the unmasked
nodes are going to be processed, so it is possible to isolate parts of a
high poly mesh using the mask and work with them in real time.

I also changed the random displacement initialization to avoid
allocating a huge amount of memory (it wasn't really necessary)

Mesh Filter: Init random displacement only once

The random mesh filter should be much faster now, and it should not do
weird things when adjusting the strength.

Mesh Filter: Initial work for relax mesh filter

The mesh filter tool now manages its own orco cache, so there is no need
for a global orco mode using the pbvh proxies. The tool should be much
faster than before.
The code for the relax mesh filter is there, but it is disabled in the
interface until the algorithm is finished.

Cleanup: comments formatting

Relax Brush: Initial implementation

Work in progress. This still has a lot of bugs and I'm not sure if this
approach is the best for this kind of smoothing. The current
implementation can't handle poles with 5 vertices or more, so it doesn't
work properly with the result of the voxel remesher. It does not support
multires.
Also, I'm using gsets to store the smoothing groups. I'm sure that there
is a better way to do it. If for some reason someone enters sculpt mode
and touches a pole with 30 vertices it should not crash, but it adds an
unnecessary overhead when working with properly constructed quad meshes.