Feature request: Honor png transparency for fulldmd images

[passionforpins]

Currently a fulldmd image will always show with a black background instead of being transparent. While it's true that windows punch cuts out the score display area so that a dmd or alphanumeric display(s) can show through the image there is no such punch for areas on the outside of the image and transparent areas are simply shown as black.
This is of course desired when using the fulldmd as intended, a separate monitor screen used solely as a full dmd. However in desktop use the black fill takes away from the oft times intricate artwork created by some authors. Imagine a scroll artwork for example.