Add Alpha support to sequences.

Alpha can specify a single value for the sequence
or values for each frame in the sequence.

AlphaFade: True can be specified to linearly fade
to transparent over the length of the animation.

OpenRA.Mods.Common/Traits/Render/WithSpriteControlGroupDecoration.cs

@@ -65,7 +65,7 @@ namespace OpenRA.Mods.Common.Traits.Render
 			var palette = wr.Palette(Info.Palette);
 			return new IRenderable[]
 			{
-				new UISpriteRenderable(anim.Image, self.CenterPosition, screenPos, 0, palette, 1f)
+				new UISpriteRenderable(anim.Image, self.CenterPosition, screenPos, 0, palette)
 			};
 		}
 	}