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 :)
Note that I voluntarily dropped that code: it might have been doing something, but it was implemented so poorly there was no way I was going to spend hours refactoring the code to keep it.

Too bad for whoever cared about EGL support on X11, he can try again to implement that in a way that doesn't make InputCommon depend on the GL backend.