Each .ttf/.ttc file may have more than one font. Each font has a sequential
index number starting from 0. Call this function to get the font offset for
a given index; it returns None if the index is out of range. A regular .ttf
file will only define one font and it always be at offset 0, so it will
return Some(0) for index 0, and None for all other indices. You can just
skip this step if you know it’s that kind of font.
Return true if data holds a TrueType Collection, according to its
signature in the initial bytes. A TrueType Collection stores several fonts
in a single file, allowing them to share data for glyphs they have in
common.
Return true if data holds a font stored in a format this crate
recognizes, according to its signature in the initial bytes.