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>

src/gallium/include/pipe/p_defines.h

@@ -425,6 +425,7 @@ enum pipe_cap {
    PIPE_CAP_OCCLUSION_QUERY,
    PIPE_CAP_TIMER_QUERY,
    PIPE_CAP_TEXTURE_SHADOW_MAP,
+   PIPE_CAP_TEXTURE_SWIZZLE,
    PIPE_CAP_MAX_TEXTURE_2D_LEVELS,
    PIPE_CAP_MAX_TEXTURE_3D_LEVELS,
    PIPE_CAP_MAX_TEXTURE_CUBE_LEVELS,