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.
I agree. But how to fix this issues else:
warning C4316: 'VertexLoader' : object allocated on the heap may not be aligned 128
I have an aligned member within the vertex loader, so also the vertex loader must be aligned. I was to stupid for this issue :/