Fix thumbnail rotation regression

This PR fixes a fairly recent regression that causes the thumbnails to not be rotated properly. The PRs responsible for the regression should be #3848, and to some extent #3978. The rotation issue might, for example, look like this:
![thumbs_rotation](https://f.cloud.github.com/assets/2692120/1809117/2bdddfec-6daf-11e3-81be-b10eb1590def.png)

In this patch, besides fixing the regression, I've also made sure that thumbnail rotations are handled the same way as page rotations. (Basically PR #3971, but for thumbnails.)

Note that the reason that I didn't add the thumbnail equivalent of this code: https://github.com/mozilla/pdf.js/blob/master/web/viewer.js#L871-L874, is that it isn't necessary and it also wouldn't help when `disableAutoFetch = true`.