xquartz: Ensure we call into TIS on the main thread

There is a place where this code was called on the main thread.

We're using a rather nasty anti-pattern to just call a block inline rather
than synchonously calling it on the main thread if we're already on the main
thread.  This code could use a good overhaul, but I don't have time to rip
it apart right now.  This will address the immediate issue.

Fixes: https://github.com/XQuartz/XQuartz/issues/40
Fixes: https://github.com/XQuartz/XQuartz/issues/48


Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
(cherry picked from commit c9a3b14c)