sixel: empty pixels in the last sixel row doesn’t contribute to the image height

All-empty pixels rows in the last sixel row should not be included in
the final sixel image.

This allows applications to emit sixels whose height is not a multiple
of 6, and is how XTerm works.

This is done by tracking the largest row number that contains
non-empty pixels.

In unhook, when emitting the image, the image height is adjusted based
on this value.

terminal.h

@@ -521,6 +521,7 @@ struct terminal {
         } state;
 
         struct coord pos;    /* Current sixel coordinate */
+        int max_non_empty_row_no;
         size_t row_byte_ofs; /* Byte position into image, for current row */
         int color_idx;       /* Current palette index */
         uint32_t *private_palette;   /* Private palette, used when private mode 1070 is enabled */