GUI: Enable font fallback on missing glyphs - #7948

PR Summary

• Enable font fallback on missing glyphs.
• Add Unifont to fonts-cjk.dat, and use it as a final fallback TTFFont.
• Update Noto Sans CJK font files to 2.004 from 1.004 to add Han character glyphs to Noto Sans KR.
  • Noto Sans KR 1.004 in fonts-cjk.dat does not have Han character glyphs.
  • Updating to 2.004 adds those glyphs to the font file.

Screenshots

I rendered this text in ScummVM, with the UI language set to Korean to force the use of Noto Sans KR, before/after the patch:

한국 韓國 Corée 鿿

• Noto Sans KR, even 2.004, does not support the last character (U+9FFF 鿿). Unifont, a fallback, must render it.

AS-IS

• Noto Sans KR 1.004 fails to render 韓國, é and 鿿.

TO-BE

• Noto Sans KR 2.004 correctly renders 韓國, é.
• Unifont, a fallback font, correctly renders 鿿.

PR Details

Font Fallback System

When a font is requested to draw a missing glyph in ScummVM, it simply omits rendering of that glyph.

• Users usually expect a fallback font, or at least a tofu (�) to appear in that case.
• It can cause an issue on CJK systems, especially Korean:
  • Korean fonts tend to support only a frequently used subset of Hangul glyphs (2350 out of 11172).
  • Modern Korean uses Han characters as a supplement, but they are often omitted, too.

This PR adds a fallback font glyph rendering system to mitigate this problem.

• Implement Graphics::Font::hasGlyph() function.
  • It returns kPresent/kAbsence when implemented.
  • It returns kUnknown on unimplemented derived classes, to prevent breaking. It is treated the same as kPresent.
• Add hasGlyph() overload to the TTFFont, a class for freetype-backed fonts.
  • No other derived font classes are touched.
• Add Graphics::FallbackFont class; an object handles multiple font instances.
  • When a glyph render is requested, it checks its fonts in order of priority using hasGlyphs() and renders the first glyph available.
  • The render layout and dimensions are fixed to the primary font; use of the fallback font is an exception.
  • When no font has a glyph, render a tofu (U+FFFD �). Render ? as a last resort when even tofu is not available.
• Add a fallback font feature to the ScummVM Theme system, as a first consumer.
  • In XML, <fonts>, <font>, <language> node can have a child node of <fallback>.

Addition/Update of fonts

This PR also adds an OpenType variant of Unifont.

• Unifont is a lightweight (~5MB) console/monospace-friendly font that aims to cover all Unicode codepoints.
• It makes a perfect candidate for a fallback font on a low-resolution, non-hinted surface.
• It suits ScummVM's license, as it is dual-licensed under the SIL OFL and the GNU Font Embedding Exception.

This PR sets Unifont as a primary fallback font on freetype-based ScummVM themes.

• Unifont was added to the CJK font dictionary to avoid a memory/storage issue on ScummVM ports for the old hardware.
• When fonts-cjk.dat is excluded from the build, the fallback system would silently disable the use of Unifont in effect.

I also updated the Noto Sans CJK font family files to 2.004 from 1.004.

• It was necessary to add Han character glyphs to Noto Sans KR.
• The current OTF file embedded in ScummVM does not contain them.

Future Work

• The ScummVM debug console is out of scope in this PR.
  • It was built on the assumption that 'every glyph has the same width', which interferes with the font fallback system.
• This fallback font system can be expanded to bitmap fonts, too.