PDFScriptingManager: Bind mousedown listener with capture=true
PDFScriptingManager: Bind mousedown listener with capture=true

PDFScriptingManager uses the `mousedown` and `mouseup` listeners to keep
track of whether the mouse pointer is pressed in the `isDown` flag.
These listeners were registered to run during the bubbling phase of the
event dispatch, which can be interrupted if any of the previous event
listeners stopped the event propagation. An example of that is by
`GrabToPan` in web/grab_to_pan.js.

Since the mousedown (and mouseup) listeners of PDFScriptingManager are
free of side effects, and the intention is to always run them, it makes
most sense to register them with the capture flag.

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