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.Mods.Common/Widgets/HueSliderWidget.cs

@@ -47,7 +47,7 @@ namespace OpenRA.Mods.Common.Widgets
 			Game.Renderer.RgbaSpriteRenderer.DrawSprite(hueSprite, ro, new float2(rb.Size));
 
 			var sprite = ChromeProvider.GetImage("lobby-bits", "huepicker");
-			var pos = RenderOrigin + new int2(PxFromValue(Value).Clamp(0, rb.Width - 1) - sprite.Bounds.Width / 2, (rb.Height - sprite.Bounds.Height) / 2);
+			var pos = RenderOrigin + new int2(PxFromValue(Value).Clamp(0, rb.Width - 1) - (int)sprite.Size.X / 2, (rb.Height - (int)sprite.Size.Y) / 2);
 			Game.Renderer.RgbaSpriteRenderer.DrawSprite(sprite, pos);
 		}
 	}