VSE strip modifier for Alpha channel

Add a VSE Strip modifier that can alter the alpha channel of the same strip by using the strip's existing colour components.

If you use a variation of the existing Hue Correct node you could make a simple chromakey effect. The modifier's output would only affect the Strip's alpha channel. Its input would be the Strip's own colours.

This is helpful for people making tutorials who cannot preprocess their camera footage to build a keyed alpha channel. It would require that the strip be allowed to use itself as a source but should also have a Boolean operation with other sources (a subtraction or addition function -normalized). Then you could make occlusions or clipping masks from other strips or Masks.