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 :)
Just to have the comment on here too (and not just on IRC): ```AVISaveOptions``` on Windows (see ```AVIDump::SetCompressionOptions```) wants a HWND to use as parent window, in case it opens a GUI. Can be left as NULL ofc, but we might want to just dump that and remove the ```#ifdef``` instead.