vulkan: Improve rounded rect clipping

Handle the case where the inner rounded rect is fully contained
within the outer rounded rect.

gsk/vulkan/gskvulkanclip.c

@@ -126,7 +126,13 @@ gsk_vulkan_clip_intersect_rounded_rect (GskVulkanClip        *dest,
 
     case GSK_VULKAN_CLIP_ROUNDED_CIRCULAR:
     case GSK_VULKAN_CLIP_ROUNDED:
-      /* XXX: improve */
+      if (gsk_rounded_rect_contains_rect (&src->rect, &rounded->bounds))
+        {
+          dest->type = gsk_rounded_rect_is_circular (rounded) ? GSK_VULKAN_CLIP_ROUNDED_CIRCULAR : GSK_VULKAN_CLIP_ROUNDED;
+          gsk_rounded_rect_init_copy (&dest->rect, rounded);
+          return TRUE;
+        }
+      /* XXX: Can be improved for the case where one of the rects is a slighty shrunk version of the other */
       return FALSE;
 
     default: