r300g: Debug flags infrastructure

r300g: Debug flags infrastructure

So that debugging is no longer a full-spam-or-nothing approach, you are now
supposed to set the RADEON_DEBUG environment flag just like for classic Mesa.

The available debug flags are different, however. Just running an OpenGL
application with RADEON_DEBUG set to an arbitrary string will print out
helpful information.

Everything must be compiled with -DDEBUG for any of this to work