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/Graphics/Sheet.cs

@@ -25,7 +25,6 @@ namespace OpenRA.Graphics
 
 		public readonly Size Size;
 		public readonly SheetType Type;
-		public float DPIScale = 1f;
 
 		public byte[] GetData()
 		{