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 :)
Huh, how is GLWin a wx dependency? Just because it lives in DolphinWX doesn't mean it's that, because then the whole NoGui build would be based on wx.

Checking the GLInterface here is precisely the correct thing to do. No other code in Dolphin knows about whether we run on X11 or on Wayland (and it doesn't make sense to let any other code than GLInterface handle it).