gpu: Avoid offscreens for disjoint containers

When opacity is set but the container is disjoint - ie no children
overlap - don't redirect into an offscreen, because it's not necessary.

gsk/gpu/gskgpunodeprocessor.c

@@ -3734,6 +3734,12 @@ static void
 gsk_gpu_node_processor_add_container_node (GskGpuNodeProcessor *self,
                                            GskRenderNode       *node)
 {
+  if (self->opacity < 1.0 && !gsk_container_node_is_disjoint (node))
+    {
+      gsk_gpu_node_processor_add_without_opacity (self, node);
+      return;
+    }
+
   for (guint i = 0; i < gsk_container_node_get_n_children (node); i++)
     gsk_gpu_node_processor_add_node (self, gsk_container_node_get_child (node, i));
 }
@@ -3773,7 +3779,7 @@ static const struct
   },
   [GSK_CONTAINER_NODE] = {
     GSK_GPU_GLOBAL_MATRIX | GSK_GPU_GLOBAL_SCALE | GSK_GPU_GLOBAL_CLIP | GSK_GPU_GLOBAL_SCISSOR,
-    0,
+    GSK_GPU_HANDLE_OPACITY,
     gsk_gpu_node_processor_add_container_node,
     NULL,
   },