mesa/st: add support for EXT_texture_swizzle.

mesa/st: add support for EXT_texture_swizzle.

This passes on r300g, the only bit I'm not really sure about is the handling
of the sampler_view in st_atom_texture.c, I unreference it there if the swizzle
value changes and I also have to create a new set of functions to create a new
one since the u_sampler.c ones don't handle swizzle so much.

adds r300g + softpipe enables, I think other drivers could pass easily enough.

Signed-off-by: Dave Airlie <airlied@redhat.com>