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 :)
Yeah, that's also a deliberate "regression". If someone really cares about the focus and parenting of the AVI compression window being right, they're free to reimplement it without having Core depend on DolphinWX.