sliders: always an even width, for centering 2-digit IDs

src/demo/sliding.c

@@ -148,6 +148,8 @@ int sliding_puzzle_demo(struct notcurses* nc){
   // we want an 8x8 grid of chunks with a border. the leftover space will be unused
   chunky = (maxy - 2) / CHUNKS_VERT;
   chunkx = (maxx - 2) / CHUNKS_HORZ;
+  // want an even width so our 2-digit IDs are centered exactly
+  chunkx -= (chunkx % 2);
   // don't allow them to be too rectangular, but keep aspect ratio in mind!
   if(chunky > chunkx + 1){
     chunky = chunkx + 1;