virtual vertex loader

This PR marks the vertex loader as virtual. It also splits the common functions into the base class, so a jit loader can also use them.

I'm not quite sure if the last commit is worth. Likely only if we add the m_current_loader as parameter for all loaders.
It would be the same for ```operator new``` and ```operator delete``` in the other file - but the old code used the same (spaces on both sides of the operator-symbol). I wouldn't mind keeping them, as ```operator <(...)``` and ```operator ==(...)``` look a bit strange to me.