VideoCommon: VertexManager -> VertexManagerBase

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.

Source/Core/VideoCommon/VideoState.cpp

@@ -54,7 +54,7 @@ static void DoState(PointerWrap &p)
 	GeometryShaderManager::DoState(p);
 	p.DoMarker("GeometryShaderManager");
 
-	VertexManager::DoState(p);
+	VertexManagerBase::DoState(p);
 	p.DoMarker("VertexManager");
 
 	BoundingBox::DoState(p);