[DirectWrite] libass doesn't always find a fallback font

[moi15moi]

In some case, DirectWrite doesn't find a fallback font, but there is actually some.

For example, I tried to display U+ABC4 with Arial, but libass display a Tofu box.
But, actually, if I specify the font Nirmala Ui, now it display it.
Note that VSFilter has the same problem (which means TextOutW has the same problem).
There is probably not any good solution. I guess we need to iterate on all the font in the IDWriteFontCollection and see if it has the glyph.

libass version: 0.17.1
windows version: 10.0.19045.4529

Ass Sample

[Script Info]
; Script generated by Aegisub 9746-feature-aa68c2e66
; http://www.aegisub.org/
Title: Default Aegisub file
ScriptType: v4.00+
WrapStyle: 0
ScaledBorderAndShadow: yes
YCbCr Matrix: None
PlayResX: 1920
PlayResY: 1080

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,Arial,20,&H00FFFFFF,&H000000FF,&H00000000,&H00000000,0,0,0,0,100,100,0,0,1,2,2,2,10,10,10,1

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:05.00,Default,,0,0,0,,ꯄ

[rcombs]

I'm not too worried since the Meitei script has been extremely rare for centuries, and only very recently started to become used again. I'd imagine this is a bug in DirectWrite's fallback code (perhaps it doesn't know that U+ABC0 through U+ABFF maps to the Mtei script ID?), and it probably affects other software as well; I'd recommend reporting this to Microsoft.

We could theoretically add a last-resort iteration over the full list, but let's at least try to bug MS about it first.

[moi15moi]

I tested with IDWriteFontFallback::MapCharacters and it also return no font, so it isn't the IDWriteTextLayout "technique" that doesn't always work.