[uniblocks] fill in background in chunk

src/demo/unicodeblocks.c

@@ -102,12 +102,15 @@ draw_block(struct ncplane* nn, uint32_t blockstart){
   int chunk;
   for(chunk = 0 ; chunk < BLOCKSIZE / CHUNKSIZE ; ++chunk){
     int z;
+    ncplane_set_bg_rgb8(nn, 8 * chunk, 8 * chunk, 8 * chunk);
     for(z = 0 ; z < CHUNKSIZE ; ++z){
       uint32_t w = blockstart + chunk * CHUNKSIZE + z;
       // problematic characters FIXME (see TERMINALS.md)
       if(w != 0x070f && w != 0x08e2 && w != 0x06dd){
+        if(ncplane_putstr_yx(nn, chunk + 1, z * 2 + 1, "  ") < 0){
+          return -1;
+        }
         ncplane_set_fg_rgb8(nn, 0xad + z * 2, 0xff, 0x2f - z * 2);
-        ncplane_set_bg_rgb8(nn, 8 * chunk, 8 * chunk, 8 * chunk);
         if(ncplane_putwc_yx(nn, chunk + 1, z * 2 + 1, w) < 0){
           return -1;
         }