If a motion event handler (or other handler running from the flush-events
phase of the frame clock) recursed the main loop then flushing wouldn't
complete until after the recursed main loop returned, and various aspects
of the state would get out of sync.
To fix this, change flushing of the event queue to simply mark events as
ready to flush, and let normal event delivery handle the rest.
https://bugzilla.gnome.org/show_bug.cgi?id=705176
If we don't dispatch the pending events then we can enter poll with events
still requiring to be processed and which can then lead to us deadlocking
there.
We can get G_IO_HUP and G_IO_IN at the same time, if the compositor writes
data to us and then closes our connection. Make sure that we dispatch events
always if we have G_IO_IN and then error out if we get G_IO_HUP after that.
When events are paused, we should not return TRUE from prepare() or check().
GTK+ handles this for events that are already in the GTK+ queue, but
we also need suppress checks for events that are in the system queue - if we
return TRUE indicating that there are events in the system queue, then we'll
call dispatch(), and do nothing. The event source will spin, and will never
run the other phases of the paint clock.
https://bugzilla.gnome.org/show_bug.cgi?id=694274
