Configurable IMAGE_DIR for annotations to get svg images to show up in Chromium extension

The Chrome extension activates PDF.js by inserting the script tags
in a document whose URL and location origin is identical to the PDF
file.
Because of this, the path './images/' was resolved relatively to the
location of the PDF file instead of the extension.

To fix this, the IMAGE_DIR constant is moved back to the global scope,
which allows extensions/chrome/insertviewer.js to override the value.

Originally, the IMAGE_DIR variable was a global variable, but commit
f8f4b3f45dc6ff5f5b2572b5387389db33616be3 moved the global variable
to the local scope, causing the extension to malfunction.

Impact: low, the only consequence is that some rarely used images
were not visible.

Trivial test:
At the center of page 2, the annotation icon
(images/annotation-comment.svg) should be visible:
http://linorg.usp.br/CTAN/macros/latex/contrib/pdfcomment/doc/pdfcomment.pdf