Finish up some of the gl_renderbuffer work.

Finish up some of the gl_renderbuffer work.
Use driRenderbuffer's offset, pitch fields in the span routines.
Remove the SetBuffer driver function.
Consolidate the code for setting CTX_RB3D_COLOROFFSET and CTX_RB3D_COLORPITCH
state in new radeonUpdateDrawBuffer() function.
Old code is surrounded by #if 000 / #endif, temporarily.