glamor: Fix accelerated rendering of GTK's ARGB vs xBGR composites.

There is some complicated code to support tweaking the format as we
upload from a SHM pixmap (aka the GTK icon cache), but if we weren't
sourcing from a SHM pixmap we just forgot to check that the formats
matched at all.

We could potentially be a little more discerning here (xRGB source and
ARGB mask would be fine, for example), but this will all change with
texture views anyway, so just get the rendering working for 1.16
release.

Fixes the new rendercheck gtk_argb_xbgr test.

v2: Squash in keithp's fix for checking that we have a non-NULL
    pixmap, and reword the comment even more.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

glamor/glamor_render.c

@@ -1038,6 +1038,16 @@ glamor_composite_choose_shader(CARD8 op,
     }
 #endif
 
+    /* If the source and mask are two differently-formatted views of
+     * the same pixmap bits, and the pixmap was already uploaded (so
+     * the dynamic code above doesn't apply), then fall back to
+     * software.  We should use texture views to fix this properly.
+     */
+    if (source_pixmap && source_pixmap == mask_pixmap &&
+        source->format != mask->format) {
+        goto fail;
+    }
+
     /*Before enter the rendering stage, we need to fixup
      * transformed source and mask, if the transform is not int translate. */
     if (key.source != SHADER_SOURCE_SOLID