gpu: Clean up the frame after we're done waiting for it

This is useful because cleaning up will do the final copies of texture
data.

It also means we use less memory, as we're going to release images that
were used in ops.

gsk/gpu/gskgpuframe.c

@@ -639,6 +639,8 @@ gsk_gpu_frame_wait (GskGpuFrame *self)
     return;
 
   GSK_GPU_FRAME_GET_CLASS (self)->wait (self);
+
+  gsk_gpu_frame_cleanup (self);
 }
 
 static void