Alpha not used when aligning

[spillerrec]

The alpha channel is ignored when calculating the difference between two images. So even if you use an alpha mask, static content such as credits can still interfere alignment. Even worse, stitches which moves a lot in both directions create a lot of transparent areas, but which is treated as solid black by the alignment algorithm.

Both Ordered and Recursive aligner needs to check if there is an alpha channel, and use it available.