Configurable y_sorting per z_index layer

[Jakub Arnold]

This is a pretty crucial missing thing, without which top down games will struggle with sprites being behind/in front of each other. Initial solution will be something like set_ysort(z_index, true/false). More sophisticated solutions may follow in the future.

[Jakub Arnold]

Fixed in 309db40, it's a bit of a slow hacky solution that'll need to be improved & we'll need benchmarks for it (#39), but for now this works.