r300g: implement fast color clear

r300g: implement fast color clear

An initial implementation made by Dave Airlie.

For it to be used, a color-only clear must be invoked and exactly one
point-sampled render target must be set. The render target must be
macrotiled (for us to overcome alignment issues) and bpp must be either
16 or 32.

I can't see a difference in performance. :(

Conflicts:

	src/gallium/drivers/r300/r300_blit.c