Apply alpha-mask to MDI images to allow for better EIT, etc. corona visibility

[Keith Hughitt]

Instead of cutting out the disk, as is done for LASCO images, everything outside of the disk could instead be cut out for images which do not have little or no features outside of the desk (e.g. MDI). This way when overlaying with EIT,etc. images, the solar corona will still have maximum visible.

JHelioviewer currently does something similar to this.