Add MrGameEng.Atlases: texture atlas builder, runtime loader and CLI tool

AtlasBuilder packs a directory tree of loose images into atlas pages plus
JSON metadata (deterministic shelf packing, incremental rebuilds, orphan
cleanup); TextureAtlas loads them back handing out Texture2DRegions, so
sprites from one page batch into a single draw call. The asset handle
generator maps .atlas files to TextureAtlas and skips page images.
Demonstrated in the sample (Assets/Atlases + 'atlas' console command),
wrapped as tools/MrGameEng.AtlasTool for build scripts.

Documented dependency exception: Atlases depends on Graphics and Assets.

samples/MrGameEng.Sample/Assets/Atlases/Textures.atlas

@@ -0,0 +1,31 @@
+{
+  "version": 1,
+  "name": "Textures",
+  "pageSize": 2048,
+  "padding": 2,
+  "pages": [
+    {
+      "file": "Textures.atlas.0.png",
+      "width": 256,
+      "height": 128
+    }
+  ],
+  "regions": [
+    {
+      "key": "player",
+      "page": 0,
+      "x": 68,
+      "y": 2,
+      "w": 64,
+      "h": 32
+    },
+    {
+      "key": "shapes",
+      "page": 0,
+      "x": 2,
+      "y": 2,
+      "w": 64,
+      "h": 64
+    }
+  ]
+}