Allow clearing backbuffer after finishing CanvasGroup

This avoids an issue where having multiple CanvasGroups overlap
would create a weird artifact

servers/rendering/renderer_rd/renderer_canvas_render_rd.cpp

@@ -1475,6 +1475,8 @@ void RendererCanvasRenderRD::canvas_render_items(RID p_to_render_target, Item *p
 			}
 
 			canvas_group_owner = nullptr;
+			// Backbuffer is dirty now and needs to be re-cleared if another CanvasGroup needs it.
+			backbuffer_cleared = false;
 		}
 
 		if (backbuffer_copy) {