Allow dragging between screens on same output

[area]

First off, thanks for this project - I was slowly working out how to do this myself when I stumbled across it, executed much better than I suspect I could!

As-is, you are prevented from switching outputs when a mouse button is held (which usually means you're dragging something). That makes sense, but you should (I believe?) be able to drag between screens on the same output; I've attempted to allow that in this PR, and it seems to be working for my purposes.

However, there are definitely some aspects I don't understand here, particularly around the various quirks on different operating systems, so happy to iterate until I do and this, or some variant of it, can be accepted. The main thing that was mysterious was that even with no modifications, I was able to drag from my secondary screen to my primary screen - I was just unable to drag from my primary screen to my secondary screen. I suspect (though it's a total guess on my part) that's something to do with the difference between desktops and screens (which I have not understood yet), but the inconsistency suggests to me something is amiss somewhere!