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.

Also:
 * Implement support for per-vertex PosMatrixIndex
 * Only update zslope constant once when zfreeze is activated.
 * Added a bunch of comments.

This avoids most of the redundant API calls.

CreateInputLayout requires a shader as an input, but it only cares about
the signature; we don't need to recompute it for different shaders with
the same inputs.

and s/\bNULL\b/nullptr/g for *.cpp/h/mm files not compiled on my machine