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