Heightmap terrain

Heightmap-based terrain for Godot 3.1 and 3.2.
It supports collision, texture painting, colouring, holes, level of detail, and grass.

Changes in 1.1.1:

– Added configuration warning if the terrain node has no data assigned
– Grey out menu items if conditions for them to work aren’t fulfilled.
– Increased maximum chunk size to 64 to help performance tuning. Default is now 32.
– Fixed `map_scale` not working
– Fixed inspector becoming blank when shader type is set to “custom” while no shader was assigned yet
– Fixed tiny imprecision when importing raw heightmaps
– Fixed version check causing the collider to be offset in Godot 3.1.2

Changes in 1.1:

– Added tool to export heightmap data to a raw file or image
– Add ability to change base density of grass
– Collision hits now report collider as the terrain node instead of null
– Most of function signatures in the API now use typed GDScript
– Fixed whitespace at the end of plugin.gd seen as an error in Godot 3.2
– Fixed collider vertically offset due to a change in the Bullet module
– Fixed inspector not updating properties when the shader is changed
– Fixed terrain not saving if the scene is saved just after a non-undoable action (using a workaround)
– Fixed terrain areas becoming black if resized bigger

Previous changelogs: https://github.com/Zylann/godot_heightmap_plugin/blob/master/CHANGELOG.md