Rework multi-resolution sprite handling:

- Sprite.Bounds now refers to rectangles in the source image.
  Use this when copying pixels, etc.
- Sprite.Size now refers to sizes in effective pixel coordinates.
  Use this when rendering.
- Sheet.DPIScale has been removed.
- "Density" term is introduced to refer to the number of artwork
  pixels per effective pixel.

OpenRA.Game/Primitives/Rectangle.cs

@@ -117,6 +117,8 @@ namespace OpenRA.Primitives
 			return rect == Intersect(this, rect);
 		}
 
+		public static Rectangle operator *(int a, Rectangle b) { return new Rectangle(a * b.X, a * b.Y, a * b.Width, a * b.Height); }
+
 		public override string ToString()
 		{
 			return string.Format("{{X={0},Y={1},Width={2},Height={3}}}", X, Y, Width, Height);