[TextLayer] Avoid unnecessary font updates in `_layoutText` (PR 11097 follow-up)
[TextLayer] Avoid unnecessary font updates in `_layoutText` (PR 11097 follow-up)

*This should obviously have been done in PR 11097, but for some reason I completely overlooked it; sorry about that.*

There's no good reason to update the font unless you're actually going to measure the width of the textContent. This can reduce unnecessary font switching a fair bit, even for documents which are somewhat simple/short (in e.g. the `tracemonkey.pdf` file this cuts the amount of font switches almost in half).

timvandermeij/pdf.js,xavier114fch/pdf.js,Shoobx/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,xavier114fch/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,caremerge/pdf.js,macroplant/pdf.js,caremerge/pdf.js,mainegreen/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,macroplant/pdf.js,nawawi/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,timvandermeij/pdf.js,mainegreen/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js