Change the `finishRenderTask` helper function, in `PDFThumbnailView.draw`, to be asynchronous
Change the `finishRenderTask` helper function, in `PDFThumbnailView.draw`, to be asynchronous

This simplifies the implementation slightly, and is also (almost) identical to the `finishPaintTask` helper function in `PDFPageView.draw`.

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