mesa: fix the compressed TexSubImage size checking code

mesa: fix the compressed TexSubImage size checking code

Before, we'd incorrectly generate an error if we we tried to
replace a non-4x4 block near the edge of a NPOT compressed texture.
For example, if the dest image was 15 texels wide and xoffset=12
and width=3 we'd incorrectly generate GL_INVALID_OPERATION.

Verified with new tests added to piglit s3tc-errors test.

Note: This is a candidate for the stable branches.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>