gallivm: generalize the compressed format soa fetch a bit

This can now handle rgtc (unorm) too - this path no longer handles plain
formats, but that's unnecessary they now all have their proper SoA unpack
(this will still be dog-slow though due to the actual fetch being per-pixel
util fallbacks).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
1 file changed