Use Heightmap Collision Shape for terrain layer colliders

[kb173]

1:1 fit of collider to terrain, generation on-the-fly

Fixes #56

[kb173]

The collision code was updated in b9c5c4d, but Godot 3.4 will bring an even better feature for this, which makes all our collider generation redundant:

godotengine/godot/pull/47349

This not only makes things easier for us, but it might also improve performance quite significantly!

[kb173]

https://docs.godotengine.org/en/stable/classes/class_heightmapshape.html

We need to get a PoolFloatArray of our heightmap, it's probably best to add this to Geodot

[kb173]

https://docs.godotengine.org/en/stable/classes/class_heightmapshape.html

We need to get a PoolFloatArray of our heightmap, it's probably best to add this to Geodot

Opened a Geodot issue for this: boku-ilen/geodot-plugin#68