[Bf-committers] Material/TexFace cleanup round 2 for texture painting

[Campbell Barton]

Suggestion...

- Keep texface behavior as is, texface may be refactored out at some
point, but until then theres no reason to remove this from
texture-paint.

- When the view-port is in GLSL/Multi-texure mode, Use the faces
materials as an image source.
... exactly how to do this isn't so straightforward, but I think using
the materials active MTex is reasonable.
... if the MTex isn't an Image texture or doesn't use UV mapping a
warning can be reported in the header.

This means the common case of painting onto some materials which use
images works by default.

The downside is when each material has diffuse/normal/spec, it could
get annoying to have to activate each material and then the MTex to
paint onto the one you want but I don't think auto-selecting the mtex
is great either since you may want to paint onto the specular images
too.