Gets the font's embedded font program, parsed by the engine's font program parsers; null when the font is
not embedded, no parser reads the program's format, or the program is unusable (each recorded as a diagnostic).
public FontProgram? Program { get; }
ISO 32000-2 §9.9: the program in the font descriptor's FontFile, FontFile2 or FontFile3 stream, decoded
through its filters. The parser is picked by the program's bytes, then by the format the stream declares (Table 124); see
IFontProgramParser. The managed defaults read TrueType and Type 1 programs.
The program is parsed once per font file stream and shared by every font and thread that uses it; it is parsed again only after the stream changes. Selecting a glyph for a character code is the font's job (§9.6.5): for a TrueType font, GetGlyphId(Byte).