How to clear a rectangle area in WebGL?
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WebGL has a `clear` method that clears an entire surface.  What's the best way to clear just a particular rectangle of the surface?  For example I want to set a 100x100 box of pixels starting at (50, 50) to all zeroes (ARGB 0, 0, 0, 0).  All I can think of right now is drawing a quad with a fragment shader that writes zeroes.  Is there not an easier way?