gpu: Consider scissor when intersecting with recangle

The clip might be different from the scissor due to incompatible
intersections.

But the resulting intersection might be fully clipped, so we should
consider it.

Testsuite with longer explanation attached.

Fixes #7044

gsk/gpu/gskgpunodeprocessor.c

@@ -1039,6 +1039,19 @@ gsk_gpu_node_processor_add_node_clipped (GskGpuNodeProcessor   *self,
     }
   else
     {
+      graphene_rect_t scissored_clip;
+
+      if (gsk_gpu_node_processor_rect_device_to_clip (self,
+                                                      &GSK_RECT_INIT_CAIRO (&self->scissor),
+                                                      &scissored_clip))
+        {
+          if (!gsk_rect_intersection (&scissored_clip, &clip, &clip))
+            {
+              gsk_gpu_clip_init_copy (&self->clip, &old_clip);
+              return;
+            }
+        }
+
       if (!gsk_gpu_clip_intersect_rect (&self->clip, &old_clip, &clip))
         {
           GskGpuImage *image;