Fix various issues with zfreeze implemntation.

Results are still not correct, but things are getting closer.

 * Don't cull CULLALL primitives so early so they can be used as reference
        planes.
 * Convert CalculateZSlope to screenspace coordinates.
 * Convert Pixelshader to screenspace coordinates (instead of worldspace
        xy coordinates, which is totally wrong)
 * Divide depth by 2^24 instead of clamping to 0.0-1.0 as was done
        before.

Progress:
 * Rouge Squadron 2/3 appear correct in game (videos in rs2 save file
         selection are missing)
 * Shadows draw 100% correctly in NHL 2003.
 * Mario golf menu renders correctly.
 * NFS: HP2, shadows sometimes render on top of car or below the road.
 * Mario Tennis, courts and shadows render correctly, but at wrong depth
 * Blood Omen 2, doesn't work.

Source/Core/VideoBackends/OGL/VertexManager.cpp

@@ -145,6 +145,10 @@ void VertexManager::vFlush(bool useDstAlpha)
 		CalculateZSlope(stride);
 	}
 
+	// if cull mode is CULL_ALL, ignore triangles and quads
+	if (bpmem.genMode.cullmode == GenMode::CULL_ALL && current_primitive_type == PRIMITIVE_TRIANGLES)
+		return;
+
 	// Makes sure we can actually do Dual source blending
 	bool dualSourcePossible = g_ActiveConfig.backend_info.bSupportsDualSourceBlend;