ximagesrc: avoid blocking wait for X events

ximagesrc: avoid blocking wait for X events

XNextEvent() blocks indefinitely in absence of X11 events, which can
prevent the pipeline from stopping.

This can cause problems when ximagesrc is used in "remote desktop"
scenarios and the GStreamer application itself, through which the user
is viewing and controlling the machine, is the only source of input
events.

Replace the call with non-blocking XCheckTypedEvent().

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1438>