[DON'T SQUASH] Implement ice breaking on falling & fix crash if ctf_map is not present

[appgurueu]

I have replaced the fall_damage_add_percent with a proper solution that works as follows:

• Check if the player fell on ice, dig the ice (with sound) if that's the case;
• If the player fell only on ice, limit the fall damage (to one heart currently, damage incurred from "breaking" the ice);
• Preserve the player velocity by calling add_velocity and inferring the speed from the damage

It has to rely on some hacks unfortunately as fall damage is entirely clientside.

A minor fix to keep ctf_map as an optional dependency is also included.

[LoneWolfHT]

ctf_map should be an optional dependency

[appgurueu]

ctf_map should be an optional dependency

Yes, that's why I fixed a crash that occurred when ctf_map was not available. I worded that weirdly in the comment, but the commit message is pretty clear.

[LoneWolfHT]

• This crashes when ground is not found. (No hp returned)
• Players still land on ice if they're moving along the x/z axis while falling
• collisionbox checking feels like overkill
• I feel like a raycast would be better than looping through minetest.get_node()

What I would try:

• Player hits ice
  • Raycast to find ground, take player velocity into cast direction
    • If player is moving
      • Remove a 3x2 (+1?) area of ice, taking the player dir into account (X = player's landing pos)
    • If player is not moving
      • Remove the ice directly below them, and set_pos() to make sure they fall through it

[appgurueu]

Raycasts work on selectionboxes which is why I have avoided them here. But I suppose we have about a year to fix this.