Xext: if a root window is given in XTestFakeInput, move to that

Xext: if a root window is given in XTestFakeInput, move to that

For absolute events, if the client specifies a screen number offset the
coordinates by that. And add a new flag so we know when _not_ to add the
screen offset in GPE.

Without this offset and the flag, GPE would simply add the offset of the
current screen if POINTER_SCREEN is set.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>