Bring back shading in texture painting.
This works now but it uses 3 texture units instead of two. Most GPUs of
DirectX 8 (OpenGL 1.4 should cover that) functionality even should have
those, but some old GPUs might not work with that. In any case, I hope
we will be moving to OpenGL 2.1 requirement soon anyway where 4-8
texture units are usually the norm.
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