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 :)
Filed https://code.google.com/p/dolphin-emu/issues/detail?id=7540&thanks=7540&ts=1407165146 so we don't forget about this.