Configurable IMAGE_DIR for annotations

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 outside the local scope,
to allow extensions/chrome/insertviewer.js to override the value.

Originally, the IMAGE_DIR variable was a global variable, but commit
f8f4b3f45d 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

extensions/chrome/insertviewer.js

@@ -108,9 +108,9 @@ function renderPDF(url) {
   var script = document.createElement('script');
   script.textContent =
   '(function(args) {' +
+  '  PDFJS.imageResourcesPath = args.BASE_URL + PDFJS.imageResourcesPath;' +
   '  PDFJS.workerSrc = args.BASE_URL + PDFJS.workerSrc;' +
   '  window.DEFAULT_URL = args.pdf_url;' +
-  '  window.IMAGE_DIR = args.BASE_URL + window.IMAGE_DIR;' +
   '})(' + JSON.stringify(args) + ');';
   document.head.appendChild(script);