OGL: implement bounding box support with ssbo

This implemention tries to be as accurate as the old SW implemention, but it will remove the dependcy of our vertexloader on videosw.

Source/Core/VideoBackends/OGL/Render.h

@@ -71,6 +71,9 @@ public:
 
 	u32 AccessEFB(EFBAccessType type, u32 x, u32 y, u32 poke_data) override;
 
+	u16 BBoxRead(int index) override;
+	void BBoxWrite(int index, u16 value) override;
+
 	void ResetAPIState() override;
 	void RestoreAPIState() override;