Waits for a vertical retrace to begin.
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
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