feat: apply ground friction on predictive velocity correction too

[sebcrozet]

Before this change, ground friction was only applied on grid cells below the ground. Now, just like grid cells below the ground induce a friction impulse proportional to the normal velocity, the grid cells above the ground now apply friction proportional to the removed normal velocity due to the predictive contact.

[Vollkornaffe]

Noticeable improvement in the 2D examples. The material doesn't seem to slide as on ice, see the screenshots below.

Before:

After:

[Vollkornaffe]

Hmm, it does look like the fluid example is worse off as the number of escaping particles is increased. I'm not sure why that is.