Anchoring issue on DirectWrite #30

From google/fonts#8100:

But the second font (Noto Serif Khmer) we can see that the (៉) drops to the lower bottom.

Thanks. The sequence is ឃ៉ា U+1783,U+17C9,U+17B6. I suspect it's another instance of a mark attachment bug I recently reported to Microsoft, but I will investigate.

What I've found is that (a) the anchors in this font are unduly complicated, which might be causing a problem, but (b) DirectWrite is also definitely doing something strange.