[aat] Adjust fallback positioning logic

Fixes https://github.com/harfbuzz/harfbuzz/issues/1528

Wish could add a test...

$ ./hb-view --font-file Thonburi.ttc --unicodes U+0E17,U+0E35,U+0E48,U+0E4A --shaper ot

                   ▃

         ▂▃▃▄▃▂▁▊        ▃  ▃       ▎
      ▗   ▅▆▆▅          ▌ ▆▆▅▆▙ ▌▗
     ▗                  ▙ ▂  ▎▗   ▟
     ▅▆▆▆▇▇▇▇▇▇▇▆▆▅▄     ▇▅▅▅ ▙▆▇
    ▁▁▂▁       ▁▁
         ▖                    ▖ ▂▂
  ▉   ▁        ▆▆        ▌   ▆  ▙▄ ▁▁
   ▙            ▉      ▃▖           ▟
      ▊         ▉      ▅▟           ▗▄
      ▊         ▉     ▗ ▏           ▇▆
      ▊    ▟    ▉      ▆             ▖
      ▊         ▉        ▌        ▂
      ▊   ▏     ▉           ▌ ▗   ▄▟
      ▊                        ▆
3 files changed