This function inverts the mask in a mask group: Everything that was transparent before becomes opaque, and vice versa.
A second application of this function restores the original condition of the mask. Nothing will be changed in the frames of the mask group.
When ungrouping a mask group, the inversion of the mask will be lost.
The write mode of the mask group is not affected by this function. It simply switches what is to be actually displayed, and not how it is displayed! The write mode is set as usual in the Frame editing module. But generally it will refer only to the portion of the mask group that is really displayed.
