Buffer Backed Terrain (for Bullet physics optimization)

[yahel-ck]

This change adds a BufferTerrain class which is exactly the same as the current Terrain class but stores the height data in a FloatBuffer rather than an array.

I use this to share the height data with the physics library (LibGDX's Bullet wrapper) instead of copying the height data which can cost a lot of CPU and RAM usage.
Sharing the height data with the current Terrain class is impossible because Bullet needs the data to be stored in a direct buffer, which the java array cannot be converted into.

I chose to implement this in a new class and not change the current Terrain class because that would be too big of a change.

The changes I made in summary:

• added a TerrainInfo interface to expose the relevant information for the TerrainMaterial without relaying on a specific terrain implementation.
• added a BaseTerrain class which is a copy of the current Terrain with an abstract getHeight method instead of a heightData array.
• made Terrain and BufferTerrain implement BaseTerrain with a backing float array/buffer.

I haven't changed anything editor related, so the editor still uses the original Terrain class and behavior is exactly the same.
I personally use this optimization for in-game procedural generation.
Using this optimization in the editor and in asset loading can be implemented in a separate PR, this change is mostly useful for direct use with the Mundus runtime/commons.