Fix interpolation in reference image generation for GLSL fragment shader tests

Emulate the barycentric coordinate interpolation done by the GPU in the
reference renderings for the fragment shader tests. Previously color
values for fragments were computed per-fragment using the bilinearly
interpolated texCoord value, which did not match up with the barycentric
interpolation from the 3 nearest vertices' color values.

This is a bug fix to the conformance suite, so the fix has been
backported as far back as version 1.0.1.
Enforce MAX_TEXTURE_SIZE to be a power of two

This is implied by GLES2.0 spec section 3.7.1. There, the expression
"level zero through k" strongly suggests that k is expected to be an
integer, and it follows that MAX_TEXTURE_SIZE should be a power of two.
This holds in practice for almost all current implementations. Clarify
this interpretation of the spec by explicitly testing it.
Merge pull request #323 from NVIDIA/interpolation-fix-clean

Fix interpolation in reference image generation for GLSL fragment shader tests
Merge pull request #324 from NVIDIA/pot-max-texture-size

Enforce MAX_TEXTURE_SIZE to be a power of two