Make vertex loader testable

This branch contains some core refactorings and global dependencies removal to make VertexLoader testable. A sample unit test has been included, and while it does not test much (a simple position conversion test) it works fine, even testing JIT :)
There is no library that includes the non DolphinWX related parts of DolphinWX, so the only way to get the global defined is to link in all of DolphinWX. This could be fixed by creating yet another library in Dolphin, or moving all this non-WX code to Core. The good way to fix this (imo) would be to have InputCommon not care about this at all and let the code that instantiates it know about what backends should be considered (move the decision upwards until we reach code that knows about this, or that could know about this with some refactoring).

If you want to fix this properly, feel free to add yourself as owner of issue 7540. We don't have a single person using Wayland regularly so I don't feel too bad by breaking it, considering this finally allows us to test some pieces of code that are badly isolated in Dolphin.

Or feel free to revert this whole change since I'm not going to spend more time on this.