text style: underline

[raphamorim]

Implement underline feature in the sugarloaf. Rio and Crosswords already provide an information if the square/cell is underlined, would be similar to the way flag is working for bold, italic and bold_italic https://github.com/raphamorim/rio/blob/main/rio/src/screen/state.rs#L193-L203

[niuez]

examples from other terminals. left is character underline, right is cursor underline. selection is to visualize glyph min and max y.

alacritty

• character underline and cursor underline is same.
• the bottoms of both underlines are same to the bottom of glyph.

• character underline and cursor underline is not same y position.
• the bottom of cursor underline is the same to the bottom of glyph.
• the bottom of character underline is placed by underline position of font information.

the implementation of alacritty is easy. but using font information is strict.

[raphamorim]

I like more Alacritty implementation (character underline and cursor underline is same), feels a bit off cursor style that specify underline do a different rendering that a normal text underline. What you think?

[niuez]

I have never used underline cursor, so I wanted to hear from anyone with an opinion.
Appearance aside, I think Alacritty's method is easy if we want just implementing underline feature. It can be changed after.

[raphamorim]

yup, we can start with alacritty's way and later revisit as well. Although might bump into this same type of problems alacritty/alacritty#3471 (if the cursor doesn't differentiate at least bit)

[raphamorim]

Thinking about it, I don't believe we are able to do the second approach until rewrite the font loader. For now implemented Strikethrough and Underline using the sugardecoration but I think we can revisit it later once we have the font loader more stable (need to rewrite it due to #80 and WASM support).

Below some screenshots with underline and strikethrough using same approach as alacritty.