ChafaSixelCanvas: Don't emit GNL after final sixel band

The final GNL could cause extra space to be emitted in some
circumstances.

Also fix an issue causing more bands to be padded than necessary when
multithreaded.

See #192 (GitHub).

chafa/internal/chafa-sixel-canvas.c

@@ -373,7 +373,6 @@ build_sixel_row_ansi (const ChafaSixelCanvas *scanvas, const SixelRow *srow, gch
     }
     while (++pen < chafa_palette_get_n_colors (&scanvas->image->palette));
 
-    *(p++) = '-';
     return p;
 }
 
@@ -393,14 +392,21 @@ build_sixel_row_worker (ChafaBatchInfo *batch, const BuildSixelsCtx *ctx)
 
     for (i = 0; i < n_sixel_rows; i++)
     {
+        gboolean is_global_first_row = batch->first_row + i == 0;
+        gboolean is_global_last_row = batch->first_row + (i + 1) * SIXEL_CELL_HEIGHT >= ctx->sixel_canvas->height;
+
         fetch_sixel_row (&srow,
                          ctx->sixel_canvas->image->pixels
                          + ctx->sixel_canvas->image->width * (batch->first_row + i * SIXEL_CELL_HEIGHT),
                          ctx->sixel_canvas->image->width);
         p = build_sixel_row_ansi (ctx->sixel_canvas, &srow, p,
-                                  (i == 0) || (i == n_sixel_rows - 1)
+                                  (is_global_first_row) || (is_global_last_row)
                                   ? TRUE : FALSE);
         chafa_bitfield_clear (&srow.filter_bits);
+
+        /* GNL after every row except final */
+        if (!is_global_last_row)
+            *(p++) = '-';
     }
 
     batch->ret_p = sixel_ansi;