Heightmap terrain

Heightmap-based terrain for Godot 4.1 and later.
It supports collision, texture painting, colouring, holes, level of detail, and grass.

This is a port of the Godot 3 version, with a few changes.

Fixes in 1.7.2:

- Fixed error when removing a texture from a TextureSet

Fixes in 1.7.1:

- Fixed terrain loading incorrectly in release (non-debug) exports

Changes in 1.7:

- Ported the plugin to Godot 4.1
- Heightmaps now use 32-bit floats instead of 16-bit floats
- Changed some APIs to use `Vector2i`
- Added `cast_shadow` setting to `HTerrain`
- Added `cast_shadow` setting to `HTerrainDetailLayer`
- Added slope limit slider to detail density painting
- Exposed `roughness` in detail layer shader (but reflections may be off due to the normals hack)
- Allow decimal values in `min_height` and `max_height` when importing a heightmap
- Changed shape list in brush settings so it can be used as a regular palette when cycling is off
- Fixed terrain not functional when using a 32-bit version of Godot (The GDNative library is only maintained for 64-bit)
- Fixed TextureSet editor logging errors when selecting slots with textures using color codes instead of image files
- Fixed texture list becoming empty when selecting another terrain sharing the same TextureSet