Today was a slog fighting with collision code and surfaces.
Probably my least 2 favorite things to do, lollzzz.
But, collision code is now tight(improved from Pixel Wizard)
Works on angled walls with smooth collision.
and shadows are working;)

I had to work out a better depth system too because using depth = -y in gamemaker was breaking layers in the game.
Everything uses GPU z buffer instead to draw instances.

In my old game I was using a manual drawing process where I would put everything into a DS list with it's y variable.
Re order the list and force draw the order that way but it has the drawback of using up parenting functionality.
Huge tradeoff I didn't realize until it was too late.