Fixes the issue that line gap may ruin the text layout.

[olliwang]

iOS uses the Hiragino Sans font as the default Japanese font. However, this font abuses the line gap property like many fonts.

• Adobe fonts usually treat line gap as the line height.
• Most other western fonts sets line gap to 200.

Back to the Hiragino font. The font family always sets line gap to 500 no matter what while the font height (ascent - descent) is only 1000. Which causes the spaces between lines are super big.

As mentioned here https://stackoverflow.com/questions/5414730/custom-installed-font-not-displayed-correctly-in-uilabel, Apple ignores the line gap property since iOS 7, and a workaround to fix this issue is to add the line gap to the ascent and we'll be fine to ignore the line gap property. Now the result looks much normal.

This change doesn't affect the alignment, either.

[memononen]

How will this affect how western multiline text is rendered? i.e. the nanovg demo.

[olliwang]

This doesn't affect western multiline at all.

PS. In my previous PR of fixing font size, the font size now matches what you saw on other native apps. The font used by the demo was affected by this because the font sizes were set to a value bigger than they actually were. I'm still fixing the font sizes and will submit another PR later.

[olliwang]

Just submitted a PR to fix demo's font size. #564

The result looks exactly the same with or without this PR's changes of fixing the line gap issue.

BEFORE:

AFTER:

[memononen]

Thanks for the fix!

[olliwang]

Thanks for the quick merging. Cheers! 🥂