GTK3: give I/O events lower priority than window redraws.

GTK3: give I/O events lower priority than window redraws.

If you run something like 'seq 2000000000' in a GTK3 pterm, the window
never actually updates, because pterm always considers reading more
data from the pty to have higher priority than delivering the "draw"
event. Using g_io_add_watch_full instead of g_io_add_watch allows us
to explicitly lower the priority of the I/O sources, so that window
redraws will take precedence.