web: Read WheelEvent.deltaMode before deltas.
web: Read WheelEvent.deltaMode before deltas.

So that Firefox doesn't switch to pixel mode for compat with other
browsers.

This should fix https://github.com/mozilla/pdf.js/issues/14476, in terms
of restoring the previous behavior.

We probably want to change the pixel-based scrolling code to not scroll
so much (the deltaMode stuff normalizes to +/-1 tick for each wheel
event, perhaps the pixel-based value should do the same).

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