This commit moves all the logic to scale up&down `<span>`s in the text
layer, introduced in #18283, to CSS.
The motivation for this change is that #18283 is still not enough for
all cases. That PR fixed the problem in Chrome&Firefox desktop, which
allow users to set an actual minimum font size in the browser settings.
However, other browsers (e.g. the Chrome-based WebView on Android) have
more complex logic and they scale up small text rather than simply
applying a minimum.
A workaround for that behavior is probably out of scope for PDF.js
itself as it only affects not officially supported platforms. However,
having access to the actual expected font height (through
`--font-height`) allows embedders of PDF.js to implement a workaround by
themselves.
eb2b7c2c86