util: Add functions that provide glOrtho-like functionality.

This patch adds functions piglit_gen_ortho_uniform() and
piglit_ortho_uniform(), which can be used to set up an orthographic
projection when the legacy function glOrtho() is not available.

The functions work exactly like piglit_gen_ortho_projection() and
piglit_ortho_projection(), except that instead of programming the
legacy projection matrix, they program an arbitrary uniform matrix.

Reviewed-by: Brian Paul <brianp@vmware.com>
util: Add compressed representations of grayscale images.

This patch adds compressed representations of uniform grayscale images
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>
Add tests for glCompressedTex{Image,SubImage}3D().

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