This adds the ability to use a secondary black-and-white video as a mask
between source A and B of the transition. The greyscale value of each
pixel is used as the "slider" value in a linear interpolation between the
corresponding pixels in source A and source B.
The track matte can either be in the same file as the stinger itself
(next to the stinger or under the stinger, doubling the width or height
of the stinger depending of the selected layout) or a in a separate
dedicated file.
The same file/separate file behavior is controlled by the
"Matte Layout" option in the stinger settings.
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