[AD] Fixed small problem with X11 acquire_screen

[Chris <kcat@xxxxxxxxxx>]

Elias noticed that there was still continuing problems with deadlocks in 
the GUI, though they were rarer. The backtrace showed that the 
mouse/timer thread was waiting on an XLOCK/acquire_bitmap while the 
bg/input thread was waiting on its own XLOCK. I traced the problem to 
_xwin_lock. I was setting the locked thread id before actually locking 
the thread, so if a context switch happened at just the right moment 
when two threads were trying to lock the screen, the program would 
become confused and deadlock.

I committed a small change, switching lines 143 and 144 of 
src/x/xvtable.c to set the locked thread id after the thread is locked, 
which seems to fix the problem.

- Kitty Cat