Add support for edgeblending

[firecraftgaming]

This PR adds a new mixer command for adding edgeblending used for overlaying projector feeds. The math is based on this article https://paulbourke.net/miscellaneous/edgeblend/. This feature could be improved so any feedback is much appreciated. The feature stems from that this is essential to our team using caspar as we need the functionality. I've only minimally tested the feature, and I have not had access to test it on real projectors yet, so any help in testing it would be much appreciated as well.