[Piglit] TBO formats testing

[Eric Anholt]

Here's the big test for TBOs: For each format, texelFetch() it in the
FS and the VS.  The test has an ARB mode for non-3.1 contexts that
tests the deprecated formats (A/I/L/LA), and core mode for the formats
that are promoted to 3.1.

Note that there's a little weirdness in the formats list: RG formats
appear in GL 3.1 core but not in either GL_ARB_texture_buffer_object
or GL_ARB_texture_rg.  So I've included them in the list of core
formats, and just don't test them if we're in a non-3.1 context.

I've now tested my implementation on i965 to almost completely work.
The only failure is that in the ARB formats,
GL_LUMINANCE_ALPHA{16,32}I (not UI or float) fail.

Testing remaining:

- TBO sampling from the default TBO or a TBO with no buffer should not
  hang or crash. (I've incidentally tested and fixed this during bugs
  in developing this test, but it should be tested long-term).

- TBO textureSize().

- TBO sampling outside of the TBO bounds.