split collisions into a separate project (#308)

Source/MonoGame.Extended.Collisions/CollisionWorldExtensions.cs

@@ -0,0 +1,17 @@
+using System.Linq;
+using MonoGame.Extended.Maps.Tiled;
+
+namespace MonoGame.Extended.Collisions
+{
+    public static class CollisionWorldExtensions
+    {
+        public static CollisionGrid CreateGrid(this CollisionWorld world, TiledTileLayer tileLayer)
+        {
+            var data = tileLayer.Tiles
+                .Select(t => t.Id)
+                .ToArray();
+
+            return world.CreateGrid(data, tileLayer.Width, tileLayer.Height, tileLayer.TileWidth, tileLayer.TileHeight);
+        }
+    }
+}