A couple of small `BaseViewer` tweaks
A couple of small `BaseViewer` tweaks

 - Mark `BaseViewer.initializeScriptingEvents` as an `async` method, since that's actually how it's being used in the default viewer (see `PDFViewerApplication-_initializeJavaScript`).

 - Change `BaseViewer._pageWidthScaleFactor` to access the *internal* scroll/spread-modes directly, rather than using the getters, since that's consistent with the rest of the code (and not just for these properties).

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