feature request: DePan alpha channel invert option

[realfinder]

Hi
DePan stable the image with adding some borders, so it will be nice if we can use the alpha plane of YUVA after we add AddAlphaPlane(255) or AddAlphaPlane(0) to normal YUV clip before feed it to DePan, so we can latter fill the borders with the help of inverted mask of the added borders that DePan added