Migrate ImageMagick from 6 to 7

[victorpopkov]

Based on the porting guide: https://imagemagick.org/script/porting.php

Seems to work fine. Tested on both:

• Linux (ImageMagick 7.1.0-5 compiled from source)
• macOS (ImageMagick 7.1.0-4 installed using Homebrew).

Should fix at least https://github.com/nsimplex/ktools/issues/7 and https://github.com/nsimplex/ktools/issues/18.

P.S. As it's very unlikely that this PR will be merged soon, so quick note for "fork investigators": this PR will be a part of my v4.5.0.

[victorpopkov]

Force-pushed into the existing branch to include some fixes regarding issues with alpha channel that I've found during in-game testing.