Followup fix for entering/exiting Presentation mode

Even after PR 5359, switching to/from Presentation mode is still sluggish and I'm still seeing `too much recursion` printed in the console.
I've managed to track down the issue to line https://github.com/mozilla/pdf.js/blob/master/web/page_view.js#L371. It appears that for this particular case, we actually *need* to set `noScroll = true` when calling the `_setScale` function. (Note that it only applies to *this* line, and not to Presentation mode in general.)
Given the amount of refactoring done to the viewer, I'm not sure what the best way to fix this would be. However I'm submitting this patch as an easy workaround for now, but we should probably refactor this to avoid calling a "private" method directly.
Merge pull request #5360 from Snuffleupagus/pr-5359-followup

Followup fix for entering/exiting Presentation mode
Add a page loading indicator to the page number input
Add updated loading-small.png (and corresponding Retina) asset