auth_pam_x11: make clearing the entire window when drawing optional.

It's now hidden behind a #define that's not enabled by default.

It simply shouldn't be necessary unless there are driver bugs, and given
auth_pam_x11 draws at low fps it's not very effective at hiding
information leaks (like the one we discovered recently involving
Cinnamon crashing) anyway. On the other hand it may use quite a lot of
CPU power, especially on low-end systems.

Effectively reverts 6b1b4f4e.