LASCO C2 transparency at the edge of field of view

[wafels]

When LASCO C2 images are set on top of C3 images, black wedges appear. This is due to rotating the data so that solar north is up. To improve the look of C2/C3 images in this situation, a transparency mask should be calculated that accounts for the rotation and masks the black wedges as being transparent. The effect of this would be to let the underlying C3 data become visible, improving the look of images and movies with this data layering.