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/SheetBuilder.cs

@@ -38,7 +38,7 @@ namespace OpenRA.Graphics
 
 		public static Sheet AllocateSheet()
 		{
-			return new Sheet(new Size(Renderer.SheetSize, Renderer.SheetSize), true);
+			return new Sheet(new Size(Renderer.SheetSize, Renderer.SheetSize));
 		}
 
 		public SheetBuilder(SheetType t)