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/VideoCommon/VertexLoaderManager.cpp

@@ -149,11 +149,8 @@ int RunVertices(int vtx_attr_group, int primitive, int count, DataReader src, bo
 	if ((int)src.size() < size)
 		return -1;
 
-	if (skip_drawing || (bpmem.genMode.cullmode == GenMode::CULL_ALL && primitive < 5))
-	{
-		// if cull mode is CULL_ALL, ignore triangles and quads
+	if (skip_drawing)
 		return size;
-	}
 
 	// If the native vertex format changed, force a flush.
 	if (loader->m_native_vertex_format != s_current_vtx_fmt)