ShroudRenderer UpdateShroud only when at least one cell is dirty.

UpdateShroud shows up in profile reports as one of the most
active methods (2.3% CPU time, main mono thread).

This commit introduces `anyCellDirty` to indicate that at lease one
of the cells was marked as dirty.

Avoiding the need to traverse all projected cells of the map
to find any dirty cell.

This reduces the number of shroud updates by at least 50% during a
test game. I assume this is related to renders occurring more
often than logic ticks(?).

OpenRA.Mods.Common/Traits/World/ShroudRenderer.cs

@@ -100,6 +100,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		readonly CellLayer<TileInfo> tileInfos;
 		readonly CellLayer<bool> cellsDirty;
+		bool anyCellDirty;
 		readonly Sprite[] fogSprites, shroudSprites;
 
 		Shroud shroud;
@@ -128,6 +129,7 @@ namespace OpenRA.Mods.Common.Traits
 			tileInfos = new CellLayer<TileInfo>(map);
 
 			cellsDirty = new CellLayer<bool>(map);
+			anyCellDirty = true;
 
 			// Load sprite variants
 			var variantCount = info.ShroudVariants.Length;
@@ -265,6 +267,7 @@ namespace OpenRA.Mods.Common.Traits
 			// Dirty the full projected space so the cells outside
 			// the map bounds can be initialized as fully shrouded.
 			cellsDirty.Clear(true);
+			anyCellDirty = true;
 			var tl = new PPos(0, 0);
 			var br = new PPos(map.MapSize.X - 1, map.MapSize.Y - 1);
 			UpdateShroud(new ProjectedCellRegion(map, tl, br));
@@ -272,6 +275,9 @@ namespace OpenRA.Mods.Common.Traits
 
 		void UpdateShroud(IEnumerable<PPos> region)
 		{
+			if (!anyCellDirty)
+				return;
+
 			foreach (var puv in region)
 			{
 				var uv = (MPos)puv;
@@ -294,6 +300,8 @@ namespace OpenRA.Mods.Common.Traits
 				shroudLayer.Update(uv, shroudSprite, shroudPos, true);
 				fogLayer.Update(uv, fogSprite, fogPos, true);
 			}
+
+			anyCellDirty = false;
 		}
 
 		void IRenderShroud.RenderShroud(WorldRenderer wr)
@@ -307,6 +315,7 @@ namespace OpenRA.Mods.Common.Traits
 		{
 			var uv = (MPos)puv;
 			cellsDirty[uv] = true;
+			anyCellDirty = true;
 			var cell = uv.ToCPos(map);
 			foreach (var direction in CVec.Directions)
 				if (map.Contains((PPos)(cell + direction).ToMPos(map)))