glamor: Fix copy-like Render operations between 15 and 16 depth.

Reading and writing to 16-depth pixmaps using PICT_x1r5g5b5 ends up
failing, unless you're doing a straight copy at the same bpp where the
misinterpretation matches on both sides.

Fixes rendercheck/blend/over and renderhceck/blend/src in piglit.

Please cherry-pick this to active stable branches.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>

glamor/glamor_render.c

@@ -512,6 +512,14 @@ static int
 compatible_formats(CARD8 op, PicturePtr dst, PicturePtr src)
 {
     if (op == PictOpSrc) {
+        /* We can't do direct copies between different depths at 16bpp
+         * because r,g,b are allocated to different bits.
+         */
+        if (dst->pDrawable->bitsPerPixel == 16 &&
+            dst->pDrawable->depth != src->pDrawable->depth) {
+            return 0;
+        }
+
         if (src->format == dst->format)
             return 1;