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/Buildings/Bridge.cs

@@ -210,7 +210,8 @@ namespace OpenRA.Mods.Common.Traits
 
 			return footprint.Select(c => (IRenderable)(new SpriteRenderable(
 				terrainRenderer.TileSprite(new TerrainTile(template, c.Value)),
-				wr.World.Map.CenterOfCell(c.Key), WVec.Zero, -offset, palette, 1f, true))).ToArray();
+				wr.World.Map.CenterOfCell(c.Key), WVec.Zero, -offset, palette, 1f, 1f,
+				float3.Ones, TintModifiers.None, true))).ToArray();
 		}
 
 		bool initialized;