xwayland: Add pointer warp emulator

Emulate pointer warps by locking the pointer and sending relative
motion events instead of absolute. X will keep track of the "fake"
pointer cursor position given the relative motion events, and the
client warping the cursor will warp the faked cursor position.

Various requirements need to be met for the pointer warp emulator to
enable:

The cursor must be invisible: since it would not be acceptable that a
fake cursor position would be different from the visual representation
of the cursor, emulation can only be done when there is no visual
representation done by the Wayland compositor. Thus, for the emulator
to enable, the cursor must be hidden, and would the cursor be displayed
while the emulator is active, the emulator would be destroyed.

The window that is warped within must be likely to have pointer focus.
For example, warping outside of the window region will be ignored.

The pointer warp emulator will disable itself once the fake cursor
position leaves the window region, or the cursor is made visible.

This makes various games depending on pointer warping (such as 3D
first-person shooters and stategy games using click-to-drag-map like
things) work.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>