GWaylandSource: Remove useless variable

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
GWaylandSource: Dispatch errors from flushing too

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
GWaylandSource: Check for poll() errors too

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
GWaylandSource: Use GLib 2.36 API

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>