Cleanup: use bool
Cleanup: redundant casts
Allow render engine to declare whether it's capable of the texture rendering or not

If render engine has bl_use_texture_preview set to truth blender wouldn't fallback to
the blender internal rendering for previews.
Fix T42106: Box image mapping shows black triangles if they point to a corner and blend is 0

After discussion with cambo here we decided it's better to choose arbitrary side of the box
(in this case it's X-axis) and use image from it. That's better than doing a blackness.

P.S. This is literally a corner case anyway.
Updating to blender master
Commit #1
Still working on it
More stuff
Moved muscle stuff away from armature files.
Added muscle_utils.c
Added make_muscleList function declaration.
More stuff
More compile fixes
Continuing update
More compile errors
More stuff
More stuff
Fixed crash on add armature
Crashes
stuff