Add video annotation support

This work is still work in progress, doesn't seem to work for http://acroeng.adobe.com/Test_Files/classic_multimedia//VolvoS40V50-Full.pdf

It does seem like it worked in the beginning, but doesn't seem to work for me.  

Rebasing change from pull request #4376 to get some reviews and pitfalls. 
This still needs to be `warn` instead (it looks like this change was lost in the refactoring).
To avoid lint errors, you will need to add `warn` to the list of globals at the top of the file: https://github.com/mozilla/pdf.js/blob/master/src/display/annotation_helper.js#L17.