Increase SheetCount back to 8.

This was previously decreased to support legacy GPUs
that only supported 8 texture image units and we need
to reserve one of these for the palette texture.

OpenGL 3.X mandates a minimum of 16 (and most most GL2
cards also supported it) so we can now safely increase
this limit.

OpenRA.Game/Graphics/SpriteRenderer.cs

@@ -18,7 +18,7 @@ namespace OpenRA.Graphics
 {
 	public class SpriteRenderer : Renderer.IBatchRenderer
 	{
-		public const int SheetCount = 7;
+		public const int SheetCount = 8;
 		static readonly string[] SheetIndexToTextureName = Exts.MakeArray(SheetCount, i => $"Texture{i}");
 
 		readonly Renderer renderer;