Well I bought it, and used the draw_angle solution. The player still jitters around everywhere and the player starts sinking into the slopes. There's soo many bugs. It feels like the player doesn't know what the angle of the ground is. The way the player is staying out of the terrian doesn't feel right at all very buggy. I turned on interpolation between pixels and it helped a little but that isn't a solution for a pixel art game. Even with that the issues with aligning with the ground still occur. The player just jitters all over the place. How can I fix it?
Have you changed the player sprite from the default? Since the sprite is used for some of the collision checking, having a different sprite (at least if it's a radically different size) could lead to changes in how the collision checking acts.
How big is the jitter / sinking? Single pixels or up towards entire tiles?
Instead of setting drawangle directly, you could try lerping towards it with angle_difference (so there's a smoother transition).
(tbh there's a lot of people that would argue that rotating sprites ruins the pixel art illusion too)