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 :)
This is ridiculous. It's your PR so you should take responsibility for all consequences. You're removing support for a feature without any sort of discussion but with the sole reason "it's poorly designed", referring to a nonexisting libdolphin which doesn't play a role currently and probably won't in the near future either.

I'm going to do the single right thing and neither condone nor revert this PR, but provide a fixup patch myself later, because admittedly I should've taken the time to review this PR earlier.