Adds basic sprite sheet support to sprites

sky/examples/game/lib/texture.dart

+part of sprites;
+
+class Texture {
+  final Image image;
+  final Size size;
+  String name;
+  final bool rotated;
+  final bool trimmed;
+
+  Rect frame;
+  Rect spriteSourceSize;
+
+  Point pivot;
+
+  Texture(Image image) :
+    size = new Size(image.width.toDouble(), image.height.toDouble()),
+    image = image,
+    trimmed = false,
+    rotated = false,
+    frame = new Rect.fromLTRB(0.0, 0.0, image.width.toDouble(), image.height.toDouble()),

[eseidel, 2015/06/23 23:22:31]

fromPointAndSize ?  I guess Image doesn't have a size accessor?

+    spriteSourceSize = new Rect.fromLTRB(0.0, 0.0, image.width.toDouble(), image.height.toDouble()),
+    pivot = new Point(0.5, 0.5);
+
+
+  Texture._fromSpriteFrame(this.image, this.name, this.size, this.rotated, this.trimmed, this.frame,
+                           this.spriteSourceSize, this.pivot) {
+  }
+
+  Texture textureFromRect(Rect rect, Point offset, bool rotated) {
+    // TODO: Implement this
+    return null;
+  }
+}