Update mag.cpp to work with wlroots 0.15.1

[marcusbritanicus]

I have not yet tested the mag plugin. I simply copied the mag.cpp from git to this branch and checked if it compiles.
If @magthe tests this, I believe we can merge. Otherwise, I will test it this weekend, after which we can merge.

[soreau]

If you run git tag, it will show you the release tag names. Here I think we want v0.7.0.

[marcusbritanicus]

I finally found a little time to test the magnifier plugin. While it "works", I am unsure if this is indeed the expected result.

https://imgur.com/a/ypxcVT2

[soreau]

Thanks.