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 :)
Let me repeat myself: The poorly implemented feature is the "render to main" one. If you're talking about removing features because of stupid implementation, talk about this feature. All of this mess here is just to support render-to-main ...