This causes the track matte render target to only be reset once per
frame, rather than potentially multiple times per frame. Palakis most
likely did not know that you're supposed to reset only once per frame in
order to prevent the render target from being rendered more than once
per frame.
(Jim note: I probably should have made texrender objects automatically
detect new frames internally so it wouldn't require this. Not Palakis'
fault.)
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