Window stream fixes
Window streaming has a couple of issues because of the various scaling factors with and without scaled-framebuffer.
One of the issue, as described in #1018 (closed), is that the video crop, based on
meta_window_actor_get_frame_bounds() values might be wrong with a scaled output.
The second issue that this MR fixes is that the clipping rectangle needs to be scaled by the buffer scale rather than the geometry scale prior to call
meta_shaped_texture_get_image(), otherwise with scaled-framebuffer enabled, we may not clip appropriately and copy too much data.
Third issue, with mutliple children, the clipping rectangle should not be scaled up, otherwise, same, we may copy too much data and overflow the buffer, and eventually crash mutter/gnome-shell when streaming a window at scale > 1.