Additional tests and fixes

[dhardy]

This PR:

Removes the highlight-range functions (already deprecated in #133) since other changes in this PR would break them.

Panic-prevention:

• Do not panic if fn text_glyph_pos matches index to more than two positions

New tests:

• Additional RTL run-breaking tests
• Shaping tests for glyph indices and wrap-point associations

Code revisions / clean up:

• Revise fn glyphs_with_effects
• Revise fn part_lengths
• Use logical order for GlyphRun::breaks and GlyphRun::glyphs (these previously used left-to-right oredr regardless of text direction)

Fixes (many confirmed through interactive testing since tests would be font-specific):

• Fix font fallback adding a wrap-point at the start of a sub-run
• Fix font fallbacks changing the font due to non-printing chars
• Fix RTL text wrapping at the wrong position
• Fix glyph-wrap-position associations for shaped (RTL) text using multiple glyphs per text index
• Fix the position returned by text_glyph_pos in shaped (RTL) text using multiple glyphs per text index
• Various other fixes for minor RTL wrapping and position issues

Behaviour changes:

• When a run is broken into sub-runs for font fallbacks, always prefer the default font face instead of the last-used face

[dhardy]

This is required by a new test. Unfortunately Rust does not allow enabling text-direction marks for specific uses only, thus we must enable it at the crate level.

[dhardy]

Although the new tests avoid depending on any font metrics, they still fail on various platforms due to font variations:

• Shaping may use multiple glyphs per text index or skip some text indices entirely (use of ligatures)
• Font fallback may cause extra breaks in text