gallium: move border color to be a color union

EXT_texture_integer also specifies border color should be a color
union, the values are used according to the texture sampler format.

(update docs)

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/docs/source/cso/sampler.rst

@@ -99,8 +99,9 @@ min_lod
 max_lod
     Maximum level of detail, used to clamp LOD after bias.
 border_color
-    RGBA color used for texel coordinates that are outside the [0,width-1],
-    [0, height-1] or [0, depth-1] ranges.
+    Color union used for texel coordinates that are outside the [0,width-1],
+    [0, height-1] or [0, depth-1] ranges. Interpreted according to sampler
+    view format.
 max_anisotropy
     Maximum anistropy ratio to use when sampling from textures.  For example,
     if max_anistropy=4, a region of up to 1 by 4 texels will be sampled.