Add CESIUM_terrain_edges extension

extensions/2.0/Vendor/CESIUM_terrain_edges/README.md

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+# CESIUM_terrain_edges
+
+## Contributors
+
+* Ian Lilley, Cesium
+* Sean Lilley, Cesium
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 specification.
+
+## Overview
+
+This extension allows terrain tiles encoded in glTF to identify vertices that make up tile edges, modeled after [quantized-mesh](https://github.com/CesiumGS/quantized-mesh). It is helpful to know which vertices are on the edges in order to add skirts to hide cracks between adjacent levels of detail.
+
+<table>
+  <tr>
+    <td>Single tile with edges highlighted</td>
+    <td>Single tile with skirts generated at runtime</td>
+    <td>Full tileset with tile coordinates superimposed</td>
+  </tr>
+  <tr>
+    <td><img src="figures/tile-edges.png"></td>
+    <td><img src="figures/tile-skirts.png"></td>
+    <td><img src="figures/tileset.png"></td>
+  </tr>
+ </table>
+
+```json
+{
+  "meshes": [
+    {
+      "primitives": [
+        {
+          "attributes": {
+            "POSITION": 0,
+            "NORMAL": 1
+          },
+          "indices": 2,
+          "material": 0,
+          "mode": 4,
+          "extensions": {
+            "CESIUM_terrain_edges": {
+              "left": 3,
+              "bottom": 4,
+              "right": 5,
+              "top": 6
+            }
+          }
+        }
+      ]
+    }
+  ]
+}
+```
+
+Each edge specifies the ID of an accessor containing indices of vertices that make up the edge. The accessor `componentType` must be `UNSIGNED_SHORT` (5123) or `UNSIGNED_INT` (5125).
+
+The four edges form a counter-clockwise ring around the triangle mesh
+
+* `left` indices are on the left edge (generally negative X) and are ordered from top to bottom (generally positive Z to negative Z)
+* `bottom` indices are on the bottom edge (generally negative Z) and are ordered from left to right (generally negative X to positive X)
+* `right` indices are on the right edge (generally positive X) and are ordered from bottom to top (generally negative Z to positive Z)
+* `top` indices are on the top edge (generally positive Z) and are ordered from right to left (generally positive X to negative X)
+
+## Optional vs. Required
+
+This extension is optional, meaning it should be placed in the `extensionsUsed` list, but not in the `extensionsRequired` list.

extensions/2.0/Vendor/CESIUM_terrain_edges/schema/mesh.primitive.CESIUM_terrain_edges.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "mesh.primitive.CESIUM_terrain_edges.schema.json",
+    "title": "CESIUM_terrain_edges glTF Mesh Primitive extension",
+    "type": "object",
+    "description": "`CESIUM_terrain_edges` extension for a primitive in a glTF model.",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "left": {
+            "allOf": [
+                {
+                    "$ref": "glTFid.schema.json"
+                }
+            ],
+            "description": "The index of the accessor containing indices that make up the left edge."
+        },
+        "bottom": {
+            "allOf": [
+                {
+                    "$ref": "glTFid.schema.json"
+                }
+            ],
+            "description": "The index of the accessor containing indices that make up the bottom edge."
+        },
+        "right": {
+            "allOf": [
+                {
+                    "$ref": "glTFid.schema.json"
+                }
+            ],
+            "description": "The index of the accessor containing indices that make up the right edge."
+        },
+        "top": {
+            "allOf": [
+                {
+                    "$ref": "glTFid.schema.json"
+                }
+            ],
+            "description": "The index of the accessor containing indices that make up the top edge."
+        },
+        "extensions": {},
+        "extras": {}
+    },
+    "required": [
+        "left",
+        "bottom",
+        "right",
+        "top"
+    ]
+}