[Button] Add Line Height feature.

[klootas]

Now that Button supports autowrap (godotengine/godot#87025 👍), it would be useful to be able to specify the line height of the wrapped label. As I was looking for it, I expected to find it under Theme Overrides/Constants.

[timothyqiu]

See also #9778

[timothyqiu]

@bruvzg Asking here as the question is not very related to the line spacing PR.

According to your explaination of how line_spacing works, it seems that currently the decender line is touching the ascender line of the text below by default (line spacing is zero).

As far as I know, fonts have their preferred line spacing information built in. e.g., Freetype has height in size metrics representing the natrual distance between baselines. Should we use this in some way when rendering multiline text? For example, make line spacing a percentage of height so that it's compatible with different font sizes.

[bruvzg]

it seems that currently the decender line is touching the ascender line of the text below by default

Yes.

As far as I know, fonts have their preferred line spacing information built in. e.g., Freetype has height in size metrics representing the natrual distance between baselines.

There's height in the font metrics, and it's usually either equal or smaller than ascent + desecnt, so we can use it to calculate default spacing, but not sure how useful it would be. Currently, we use processed line ascent + descent (calculated from all used fonts, and adjusted to fit offset glyphs, like diacritics) and not values from the font.