Clip Masks

A grey-scale image that can be assigned to a clip, which hides part of the clip, and shows through part of the clip. The user should be able to modify the following properties:

1) softness (hard line or soft line)
2) progress (from black to white)

This will work much like the luma transitions (i.e. grey-scale wipes), except they won't wipe. They will be fixed at some point, and simple be used as a mask.