mesa: implement per-buffer color masking

mesa: implement per-buffer color masking

This is part of the GL_EXT_draw_buffers2 extension and part of GL 3.0.

The ctx->Color.ColorMask field is now a 2-D array.  Until drivers are
modified to support per-buffer color masking, they can just look at
the 0th color mask.

The new _mesa_ColorMaskIndexed() function will be called by
glColorMaskIndexedEXT() or glColorMaski().