mesa: Add missing format unpack for some integer texture formats.

mesa: Add missing format unpack for some integer texture formats.

This cut and paste is pretty awful.  I'm tempted to do a lot of this
using preprocessor tricks for customizing the parameter type from a
template function, but that's just a different sort of hideous.

Fixes 8 Intel oglconform int-textures cases.

NOTE: This is a candidate for the 8.0 branch.
v2: Add alpha formats, too.

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