The boolean that the server gives us for whether the region is tiled was
getting used as the enum for what tiling mode. Instead, guess the correct
tiling in screen setup.
Also, fix the Y-tiling pitch setup. The pitch to the next tile in Y is
32 scanlines, not 8.
Each array element is now a BUFFER_x token rather than a BUFFER_BIT_x bitmask.
The number of active color buffers is specified by _NumColorDrawBuffers.
This builds on the previous DrawBuffer changes and will help with drivers
implementing GL_ARB_draw_buffers.
Putting the bufmgr in the screen is not thread-safe since the emit_reloc
changes. It also led to a significant performance hit from pthread usage
for the attempted thread-safety (up to 12% of a cpu spent on refcounting
protection in single-threaded 965). The motivation had been to allow
multi-context bufmgr sharing in classic mode, but it wasn't worth the cost.
