vulkan: fix a clip handling problem

We were looking at uninitialized memory here, instead
of the type of the source clip, as we should.

This showed up as mispositioned clip in the first frame
of a crossfade stack transition, and also as overdraw in
sliding stack transitions.

gsk/gskvulkanclip.c

@@ -133,7 +133,7 @@ gsk_vulkan_clip_transform (GskVulkanClip           *dest,
                            const graphene_matrix_t *transform,
                            const graphene_rect_t   *viewport)
 {
-  switch (dest->type)
+  switch (src->type)
     {
     default:
       g_assert_not_reached();