Minor
Minor
[fallback] Add TODO note
[coretext] Avoid font fallback with CoreText shaper

CoreText does automatic font fallback (AKA "cascading") for  characters
not supported by the requested font, and provides no way to turn it off,
so detect if the returned run uses a font other than the requested one
and fill in the buffer with .notdef glyphs instead of random indices
glyph from a different font.