Lazily generate buffer in Sheet.

The managed byte buffer is created on demand, meaning a newly allocated sheet will not waste memory holding onto the buffer until some changes are actually required to be written. This avoids a newly allocated sheet wasting memory on buffers that do not differ from their backing texture.

OpenRA.Game/Graphics/Theater.cs

@@ -33,7 +33,7 @@ namespace OpenRA.Graphics
 					throw new SheetOverflowException("Terrain sheet overflow. Try increasing the tileset SheetSize parameter.");
 				allocated = true;
 
-				return new Sheet(new Size(tileset.SheetSize, tileset.SheetSize), true);
+				return new Sheet(new Size(tileset.SheetSize, tileset.SheetSize));
 			};
 
 			sheetBuilder = new SheetBuilder(SheetType.Indexed, allocate);