Fix rendering of denormalized strings with diacritics

[mourner]

Closes #7195. Modern browsers (everything except IE and Chrome in Android WebView) have a String normalize method which combines characters where possible so that ones with diacritics would be a single code point rather than two. This in turn makes it request the right glyphs for proper rendering of denormalized strings with diacritics:

Before:

After:

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality (probably needs a render test?)
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fix rendering of denormalized strings with diacritics</changelog>

[ryanhamley]

This is great. Can we add a render test for this?