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/drivers/softpipe/sp_screen.c

@@ -90,6 +90,8 @@ softpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
       return 1;
    case PIPE_CAP_TEXTURE_SHADOW_MAP:
       return 1;
+   case PIPE_CAP_TEXTURE_SWIZZLE:
+      return 1;
    case PIPE_CAP_MAX_TEXTURE_2D_LEVELS:
       return SP_MAX_TEXTURE_2D_LEVELS;
    case PIPE_CAP_MAX_TEXTURE_3D_LEVELS: