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.Game/Graphics/SequenceProvider.cs

@@ -38,6 +38,7 @@ namespace OpenRA.Graphics
 		Sprite GetSprite(int frame);
 		Sprite GetSprite(int frame, WAngle facing);
 		Sprite GetShadow(int frame, WAngle facing);
+		float GetAlpha(int frame);
 	}
 
 	public interface ISpriteSequenceLoader