Make shadow palette of GravityBomb configurable

OpenRA.Mods.Common/Effects/GravityBomb.cs

@@ -31,6 +31,8 @@ namespace OpenRA.Mods.Common.Effects
 
 		public readonly bool Shadow = false;
 
+		[PaletteReference] public readonly string ShadowPalette = "shadow";
+
 		public readonly WDist Velocity = WDist.Zero;
 
 		[Desc("Value added to velocity every tick.")]
@@ -95,7 +97,7 @@ namespace OpenRA.Mods.Common.Effects
 				{
 					var dat = world.Map.DistanceAboveTerrain(pos);
 					var shadowPos = pos - new WVec(0, 0, dat.Length);
-					foreach (var r in anim.Render(shadowPos, wr.Palette("shadow")))
+					foreach (var r in anim.Render(shadowPos, wr.Palette(info.ShadowPalette)))
 						yield return r;
 				}