Blender Git Loki

Sculpt vertex colors: Initial implementation

This adds a new CustomDataLayer to store per vertex color data in a way
that can be used from sculpt mode. This vertex paint mode uses the same
brush, pbvh, undo and rendering code as sculpt mode, so it has much
better performance. It also enables other features like sculpting and
painting at the same time.

I also added some extra features that are needed to start testing this
paint mode, such as:
- New paint brush optimized for painting. It doesn't modify any vertex
position data.
- Sculpt mode sample color operator that uses the active vertex from the
cursor
- Remesh reprojection support: sculpt vertex colors are reprojected into
the new mesh when using the remesh operator if the mesh has that option
enabled.
- Store and load sculpt vertex colors into the regular vertex color
layers to keep compatibility.
- Use the single color display mode to disable sculpt vertex colors in
sculpt mode.

A lot of decisions needs to be made to integrate this in Blender, so I
decided to keep the implementation as unintrusive as possible to start
working on the tools. This has the following limitations:

- Sculpt and paint at the same time is only enabled in the draw brush.
You need to set the Color Mode to Mix in the brush options.
- Dyntopo and multires modes and viewport display are broken
- Only mix as a color blend mode
- No brush texture support
- Sculpt and color share the same strength
- Limited to one layer per object
- No rendering support. You will need to copy the sculpt vertex colors
to the old color layers first.

All of these issues should be addressed once the main workflow, final UI
and behavior are decided.