The binning pass sorts vertices into which bins/tiles they apply to.
The visibility information generated during the binning pass can be
used to speed up the rendering pass by filtering out vertices which
do not apply to the current tile.  See:

 https://github.com/freedreno/freedreno/wiki/Adreno-tiling#optimized-approach



This brings a significant fps boost.  A rough assortment of tests
(supertuxkart, etracer, tremulous, glmark2 'build' test, etc) seems
to yield a ~35-45% fps improvement.

For now, to be conservative, the binning pass is not enabled yet by
default.  To enable it use:

  FD_MESA_DEBUG=binning

So far I haven't found anything that breaks with binning enabled,
but I'd like a bit more testing before I enable it as default.

Signed-off-by: Rob Clark <robclark@freedesktop.org>