shaped-texture: Make setting the same texture a no-op

Will be helpful when pushing state to the shaped texture, letting the
one pushing not have to care about checking if anything changed.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/961
(cherry picked from commit 3969285e5c)

src/compositor/meta-shaped-texture.c

@@ -932,6 +932,9 @@ meta_shaped_texture_set_texture (MetaShapedTexture *stex,
 {
   g_return_if_fail (META_IS_SHAPED_TEXTURE (stex));
 
+  if (stex->texture == texture)
+    return;
+
   set_cogl_texture (stex, texture);
 }