Mir on X does not grab the x11 cursor when the surface is confined

To reproduce:
1: Start a Mir On X session
2: Run an example/demo that confines the pointer to a surface (so far only an SDL2 example exists)
3: Moving the pointer to the left as far as you cant

Expected:
1: The mouse continues to generate events for the confined surface

Results:
1: The X11 mouse moves out of the Mir On X window and all mir events stop generating.

A possible fix would be to imagine the X11 platform like a WM window. It grabs the pointer so you cant move either mouse outside of the X11 boundaries and you need a key combo to release the grab.

Not sure if this is wanted or not but without you cannot use anything that depends on pointer confinement in mir on Mir On X.