GLX: Fix a potential deadlock in OnDisplayClosed.
This is a fix for the deadlock reported in #213 (closed).
There's a lock order inversion between glXMakeCurrent
(which locks the context hashtable, then the display hashtable) and the OnDisplayClosed
callback (which locks the display hash, then the context hash).
To fix that, this changes the OnDisplayClosed
callback so that it will do all of its display cleanup stuff without sitting on the display hashtable lock.