Add terrarium encoding for terrain.

Author: brawer

spec/v3/CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+## Version 3.6
+- Add `terrarium` encoding for terrain.
+
 ## Version 3.5
 - add [MapLibre Vector Tile](https://github.com/maplibre/maplibre-tile-spec) `.mlt` enum by @msbarry [#596]
 
@@ -18,4 +21,4 @@
 ## Version 3.1
 - added `metadata` details about `vector_layers`.
 - Clarified that directory entry lengths must be nonzero, and directories must be non-empty.
-- add AVIF to TileTypes.
+- add AVIF to TileTypes.

spec/v3/spec.md

@@ -374,6 +374,7 @@ Additionally, this specification defines the following keys, which MAY be includ
 |`attribution`|An attribution to be displayed when the map is shown to a user. Implementations MAY decide to treat this as HTML or literal text. |string|
 |`type`|The type of the tileset |a string with a value of either `overlay` or `baselayer`|
 |`version`|The version number of the tileset|a string containing a valid version according to [Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html) |
+|`encoding`|Any additional encoding applied to tile data. A value of `terrarium` means the tileset is a Digital Terrain Model with elevation in meters equal to `(red * 256 + green + blue / 256) - 32768`.|string|
 
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