Make source code actually readable & buildable

[Galster-dev]

Changes:

• Use submodules instead of cloned repos as doing otherwise is against a lot of licenses[1]
• Use CMake instead of Visual Studio solution files as it's much easier to build and distribute
• Fix most of unneeded warnings and copies
• Change gd.h submodule url[1]
• Make build docs
• Fix obvious code flaws and bugs

[1] You seem to have your own gd.h modifications. I've forked gd.h for this pull request and I'm fine to move ownership to this org