Text layer moves up during search

[Rob--W]

1. Visit the following PDF (http://movelex.hu/fenyoapa/tori_6_tk_webre.pdf_150.pdf) with the text layer enabled, e.g. using the URL parameter:http://movelex.hu/fenyoapa/tori_6_tk_webre.pdf_150.pdf#textLayer=visible
2. Look at the text below the top-left image (looks all right).
   
3. Use the search box and search for "pesti".
4. Look at the text below the top-left image, and observe that the text layer has shifted up.
   

I've found this bug and PDF at https://stackoverflow.com/questions/25922247/how-to-fix-pdf-js-find-highlight-error-or-what-causes-it.

[Rob--W]

The shifting is caused by the invocation of scrollIntoView in TextLayerBuilder.renderMatches. The bug is caused by the fact that the PDF document contains text outside the visible view of the page (which becomes obvious if you add overflow:visible to the textLayer):

A way to work around the issue is to skip elements whose overflow attribute is set to hidden, which I've done in the next patch. Even though the patch resolves the issue, I wonder why we have text outside the page's view at all.

[Hengjie]

Perhaps we should prevent an element to be added if they exist outside of the textLayer's dimensions.