Cleanup and refactor of zfreeze port

Based on the feedback from pull request #1767 I have put in most of
degasus's suggestions in here now.

I think we have a real winner here as moving the code to
VertexManagerBase for a function has allowed OGL to utilize zfreeze now
:)

Correct use of the vertex pointer has also corrected most of the issue
found in pull request #1767 that JMC47 stated.  Which also for me now
has Mario Tennis working with no polygon spikes on the characters
anymore!  Shadows are still an issue and probably in the other games
with shadow problems.  Rebel Strike also seems better but random skybox
glitches can show up.

Source/Core/VideoCommon/VertexManagerBase.h

@@ -41,6 +41,8 @@ public:
 
 	static void DoState(PointerWrap& p);
 
+	static void CalculateZSlope(u32 stride);
+
 protected:
 	virtual void vDoState(PointerWrap& p) {  }