Function Swirl for vctrans vapoursynth Plugin

#
Swirl

The transition from Clip A to Clip B occurs by Swirling of A merging with swirl of B frame and then subsiding the swirl to reveal B.

## trans.Swirl(clip, clipb clip[, float overlap = 1.0 ][, int dir = 1][, int qr = 3] )

Parameter description:
clip : A clip. Constant format.
clipb : B clip. Must have identical format of A clip
overlap : transition time length in seconds. Floating point. Must correspond to at least 8 frames. A longer duration is preferable.
dir :Swirl direction 0 or 1. Clockwise or anticlockwise
qr : Quality 1 to 5. 1 fine 5 coarse
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