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/WithCargoPipsDecoration.cs

@@ -92,7 +92,7 @@ namespace OpenRA.Mods.Common.Traits.Render
 			for (var i = 0; i < pipCount; i++)
 			{
 				pips.PlayRepeating(GetPipSequence(i));
-				yield return new UISpriteRenderable(pips.Image, self.CenterPosition, screenPos, 0, palette, 1f);
+				yield return new UISpriteRenderable(pips.Image, self.CenterPosition, screenPos, 0, palette);
 
 				screenPos += pipStride;
 			}