Bugs in dual screen mode

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DISCLAIMER: I am mainly creating this issue for documentation purpose, as I am currently working on a fix for the described problems and am planning on submitting a PR soon

I have recently started using snaptile again and have come across two bugs in dual screen mode (-d):

1. The position is calculated based on the current screen; therefore, eg. pressing ctrl+alt+d moves the window between the screens on multiple presses. As a result, offsets on the second screen are wrong most of the time
2. If the screens are not aligned / different sizes / ..., the calculated position is wrong in a lot of cases, since only one screen is used to calculate offsets (which are then wrong on the second screen). Of course, positioning windows spanning multiple, misaligned screens is not trivial; I would suggest to just calculate the two quadrants on the respective screen (the two corners) and drawing a rectangle around them; this means that the resulting geometry might be a bit weird sometimes (reaching out of the screen...), but would at least be consistent with the system.