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/Fifo.cpp

@@ -367,7 +367,7 @@ void RunGpuLoop()
 
 			// The fifo is empty and it's unlikely we will get any more work in the near future.
 			// Make sure VertexManager finishes drawing any primitives it has stored in it's buffer.
-			VertexManager::Flush();
+			VertexManagerBase::Flush();
 		}
 	}, 100);