Convert `src/display/svg.js` to ES6 syntax
Convert `src/display/svg.js` to ES6 syntax

In particular, this should reduce intermediate string creation by using
template strings and reduce variable lookup times by removing unneeded
variables and caching `this.current` in more places.

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