Add user-definable masks

An additional track mask would be helpful which allows the user to specify a grayscale file which determines the opacity of the track - similar to GIMP's layer mask (white = full opacity, black = full transparency).

This feature could even be extended to an image sequence of grayscale files to change the track's opacity over time.

Such a feature would allow e.g. to blur a certain region (e.g. a face), to desaturate objects and so on. Many things which are done nowadays using GAP could maybe done easier using these masks.