More XErrors traps

Previously we would leak error traps in free_win_data() causing
slowdowns overtime as the list of requests grows.

While fixing the leak is crucial, it also unveiled several places where
actual error traps are missing, causing xfwm4 to abort more often.

Based on XErrors reported, the issue is triggered when freeing pixmaps
or render pictures.

Add more XErrors traps to catch those errors.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
#433
#435
#436