Add option to scale images in RichTextLabel relative to font size

[Meorge]

Closes godotengine/godot-proposals#13609.

This PR allows img tags in RichTextLabels to specify height and width with the em suffix, which makes the values a percentage of the surrounding text's font size.

For example, the following text:

Do you have any [img height=1em]coin.png[/img] coins?
...I said, [font_size=50]DO YOU HAVE ANY [img height=1em]coin.png[/img] COINS??[/font_size]

Displays like this:

[Calinou]

with the em suffix

I'm curious if we should go with rem (root em) instead. Last time I checked, many people considered em in CSS to be a design flaw, which is why most people use rem in CSS nowadays. There are still a few legitimate reasons to use em in CSS in 2025, but rem is more suited in most cases.

That said, maybe it's not a relevant distinction here.

[Meorge]

If I'm understanding rem correctly, it sounds like it wouldn't respect [font_size] BBCode blocks. That is to say, if you had the text

I said, [font_size=50]DO YOU HAVE ANY [img height=100rem]coin.png[/img] COINS??[/font_size]

and the theme/default font size for this RichTextLabel is 16px, then the coin.png sprite would be 16px tall, even though it's in a block of text that's 50px tall.

I suppose we could include both rem and em; the latter would respect [font_size] whereas the former wouldn't. I don't know how many use cases there would be for that, though.

[Meorge]

The 1em measurement is equal to 100% of the surrounding font size, rather than 100em.

This required changing the width and height values from ints to floats, so now for free the pixel sizes and percentages also support float values!