vsync - Waits for a vertical retrace to begin. Allegro game programming library.
Waits for a vertical retrace to begin. The retrace happens when the electron
beam in your monitor has reached the bottom of the screen and is moving back
to the top ready for another scan. During this short period the graphics card
isn't sending any data to the monitor, so you can do things to it that aren't
possible at other times, such as altering the palette without causing
flickering (snow). Allegro will automatically wait for a retrace before
altering the palette or doing any hardware scrolling, though, so you don't
normally need to bother with this function.