http://octavoarte.cl/25d_concept.exe
Just a concept. No slopes. Seems to work allright, you may even stretch objects and they still work
There's a marker for the player's ground level (w = 0)
Platforms can be moved vertically (not done in demo)
Edit: here it's sorting by basal y (ry) only. Final sorting must involve height somehow, haven't figured it out yet. In this demo, if you go towards the back of a platform, you'll be drawn behind it. A temporary workaround was to set the hotspot to the top of the base collision mask.
TODO/bugs:
-falling faster than maxclimb on a platform lets you through it
-platforms below others don't let you step on the top one
-there's no hitting walls yet
-integrator is very naive. Go Verlet.
-consider having the mask at the top instead of the bottom--> easier to draw
-move platforms on rx or ry, with other objects resting on them?