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 :)
I'm pretty sure the point is that EGL is mandatory on Wayland, so an X11 check needs to be done here.
If `GLWin.platform` is available in the non-EGL code path, the preprocessor tokens can simply be dropped and the `if` statement always be enabled. Otherwise, a comment should be added that when using EGL, chances are we are on Wayland.

In any case, I don't support such frivolous dropping of features (or compatibility with nonstandard software configurations, depending what you call it). It's not really implemented poorly, if anything the code within the `if` statement is completely out of place here to begin with.