Don't round font sizes

[jaredkhan]

Fixes #899

This is a pretty big improvement for basically no effort so maybe go with this for now.
(See video demos on the issue #899)

There's still a bit of jitteriness of multi-line text but I think that's more of a limitation of the canvas API: It doesn't seem to let you write text whose baseline isn't on a pixel boundary.

[hyanwong]

LGTM. Nice that it's so easy. I wonder why we rounded in the first place?

[jaredkhan]

Yeah it is a bit curious. I did have half a think about it:

I found this StackOverflow from 11 years ago talking about how browsers seemed to round font sizes in canvas:
https://stackoverflow.com/questions/15430763/fractional-font-sizes-html5-canvas

and in particular they produced this graphic of incrementing font sizes by 0.1px and the resulting output

but when I tried to produce a similar graphic in 2024 I didn't find such agressive rounding, though there was still some in Firefox

left-to-right: Firefox, Safari, Chrome.

I also found that it does seem to be way better on Firefox to scale text using the ctx.scale rather than using the fontsize.
In this graphic, the stack on the left is using font size, the stack on the right is using ctx.scale. The rectangles are boxes drawn with the result of a measureText call on that text. So you can see that the text size rounding is gone and the measuretext is more accurate when using ctx.scale.

but that's a bit more of a faff to change for less of a return, hence suggesting this PR in the end.

So perhaps the rounding was based on previous typical behaviour of canvas that's no longer so typical?
Or perhaps it was just an assumption that whole numbers were required.
Or perhaps I'm missing something else entirely.