Blender Git Loki

Support for textures in vertex painting.

Developer notes: this commit does painting in sRGB space. Since colours
are stored im byte per component formats, expect this to have the usual
dark fringing issues. Speed wise vertex paint could use some
optimization, for instance we could store the screen space vertex
positions on initialization like we do for texture painting, but this is
for another time. Also noticed that vertex painting suffers from the
subsurf + mirror issue too :/

Apart from that it's quite exciting how easy it is to add support for
texturing now that proper abstractions for texture sampling have been
done :)