When using a stacked track matte stinger transition with a visibility
transition, the matte portion of video would also be visible on the
right side or below the source. This fixes it by first rendering the
video to a texrender object in order to filter out the unwanted portion.
(Jim note: So basically, track matte stingers now use two texrenders. I
really don't like this. Texrender objects can already cause stalling as
it is.)
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