[Piglit] Slight fixes/enhancements for handling depth 30 / 10 bpc rendering.

[Mario Kleiner]

Hi,

so i just sent out a patch series to mesa-devel to add
XRGB2101010/ARGB2101010 fbconfig/visuals/rendering support
to the Intel i965 driver. The patches here try to fix observed
piglit failures with the piglit "gpu" subset of tests, as far as i
think they are false positives.

Patch 1 and 2 fix false positives when running under a depth 30
x-screen or with RGBA 10 10 10 2/0 visuals exposed under glx/egl.

About patch 3 and 4 i'm not sure if adding them makes sense? Without
patch 3, running with color depth 30 / 10 bpc, causes piglit to skip
a couple of tests. Adding the patch will make it more accepting, but
then many tests fail because they assume the alpha channel has at
least 8 bits, an assumption that is violated in 10 bpc rendering with
only 2 bits of alpha. Patch 4 fixes some of those false failures, but
not all of them. I'm also not sure if overriding the alpha bits inside
piglit_set_tolerance_for_bits() is really the right approach for all
tests. Maybe just skipping sets of tests for the depth 30 rendering is
more sensible?

I don't have commit access, so if some of these get accepted, somebody
else has to apply them.

Thanks,
-mario