util: Add compressed representations of grayscale images.

This patch adds compressed representations of uniform grayscale blocks
to piglit, in the ETC1 and FXT1 compression formats.  Although they
don't exercise the full capabilities of these compression formats,
they should be sufficient to create basic tests of compression
functionality without having to rely on the GL implementation to do
compression on the fly.

Reviewed-by: Brian Paul <brianp@vmware.com>
Acked-by: Eric Anholt <eric@anholt.net>

v2: Add S3TC.
Add tests for glCompressedTex{Image,SubImage}3D().

Reviewed-by: Brian Paul <brianp@vmware.com>

v2: Switch the desktop GL test to use S3TC so that the test can run on
a greater variety of hardware.
WIP: basic varying struct test