[Piglit] Core context tests and ARB_compatibility

[Alexander Monakov]

Hello,

I think there is a problem in piglit implementation that "breaks" some
tests on nvidia proprietary drivers, for instance "textureSize 140 fs
sampler2D -auto -fbo" produces:

piglit: info: Failed to create GL 3.1 core context
GLSL 1.40 not supported.

Stepping through the test in gdb I see that it first acquires a 3.1
core context, but decides to throw it away since it has
ARB_compatibility. Then the tests asks for a 1.0 compat context, and
gets a 2.1 context with glsl-1.20.

I'm told this test somehow works on amd proprietary drivers.

Alexander