Gets rid of some indirect inclusions in cpp files.
Also this will reduce the amount of rebuilt files if
changes occur in the DataReader header.

Also remedies places where the video backends and core rely on things
being indirectly included.

Now we require lots of empty functions, but this removes by far more duplicated code.

They were only called at once, so no need to seperate them.

This also removes the only dereference of the NativeVertexFormat in VideoCommon, so backends may just return nullptr.

It may be a bit weird to see calls to static functions in
VertexManagerBase now, but at least it's easier to see what's going on.