Alpha (masking) display output

For external video mixing, an alpha channel is very useful. In one case, the text can be overlaid on a video background. Another case is picture-in-picture, where the OpenLP output can disappear when the blanked (https://www.youtube.com/watch?v=M4Dk_zx07PI). This feature would require a third screen output that matches the second screen. The output on this would be a greyscale image where black means transparent and white means opaque. It is proposed that the alpha channel output could have two separate options (check boxes):
1. transparent alpha (whole window) when blank (not blank to desktop or blank to theme)
2. transparent alpha where background is transparent