Threaded swap wait was added for using together with the Nvidia GLX
driver due to the lack of anything equivalent to the INTEL_swap_event
GLX extension. The purpose was to avoid inhibiting the invocation of
idle callbacks when constantly rendering, as the combination of
throttling on swap-interval 1 and glxSwapBuffers() and the frame clock
source having higher priority than the default idle callback sources
meant they would never be invoked.

This was solved in gbz#779039 by introducing a thread that took care of
the vsync waiting, pushing frame completion events to the main thread
meaning the main thread could go idle while waiting to draw the next
frame instead of blocking on glxSwapBuffers().

As of GNOME/mutter!363, the
main thread will instead use prediction to estimate when the next frame
should be drawn. A side effect of this is that even without
INTEL_swap_event, we would not block as much, or at all, on
g...