A few fixes and enhancement related to bold text:
- When using bold=true with a regular font, use its bold
variant if one exists (can be prevented by manually
adding a setting: "use_bold_font_for_bold" = false).
- When using a bold font without bold=true, promote bold
to true, so fallback fonts are drawn bold too.
- Whether using a bold font, or using bold=true, ensure
fallback fonts are drawn bold, with their available bold
variant if one exists, or with synthetized bold.
- When using a bold variant of a fallback font, keep using
the regular variant as another fallback (as bold fonts
may contain less glyphs than their regular counterpart).
- Allow providing bold=Font.FORCE_SYNTHETIZED_BOLD to
get synth bold even when a bold font exists (might be
interesting to get text in bold the same width as the
same text non-bold).
- Use the font realname in the key when caching glyphs,
instead of our aliases (cfont, infont...) to avoid
duplication and wasting memory.
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