glmSetSwapInterval Set number of video frames between buffer flushes
usage: glmSetSwapInterval( nframes )
Note: The AGL documentation indicates that 'nframes' will control the
number of vertical refreshes between buffer flushes. In practice, it seems
that nframes=0 disables vertical refresh syncing, and nframes=1 or higher
just enables it.
Psychtoolbox/PsychOpenGL/MOGL/wrap/glmSetSwapInterval.m