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/BPFunctions.cpp

@@ -23,7 +23,7 @@ namespace BPFunctions
 
 void FlushPipeline()
 {
-	VertexManager::Flush();
+	VertexManagerBase::Flush();
 }
 
 void SetGenerationMode()