compositor: Don't select for stage events on the UI X11 connection
backend-x11: Set the proper coordinates on the spoofed device event

When we click on a window with a passive grab, then the event_x
and event_y will be relative to that window, instead of relative to
the stage, which means that picking will be wrong.

Forcibly using root_x / root_y breaks nested mode. Nested mode is
a testing mode that should be replaced by a DRI3-enabled Xephyr,
though. It's getting too hairy to support properly.
backend-x11: Split out event translation to another method

Keep the indentation from getting too hairy.