Fixes symbols positioning in JBIG2

src/jbig2.js

@@ -823,7 +823,7 @@ var Jbig2Image = (function Jbig2ImageClosure() {
         textRegion.transposed = !!(textRegionSegmentFlags & 64);
         textRegion.combinationOperator = (textRegionSegmentFlags >> 7) & 3;
         textRegion.defaultPixelValue = (textRegionSegmentFlags >> 9) & 1;
-        textRegion.dsOffset = (textRegionSegmentFlags >> 10) & 31;
+        textRegion.dsOffset = (textRegionSegmentFlags << 17) >> 27;
         textRegion.refinementTemplate = (textRegionSegmentFlags >> 15) & 1;
         if (textRegion.huffman) {
           var textRegionHuffmanFlags = readUint16(data, position);

test/pdfs/issue2642.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=707492

test/test_manifest.json

@@ -489,6 +489,14 @@
       "link": true,
       "type": "eq"
     },
+    {  "id": "issue2642",
+      "file": "pdfs/issue2642.pdf",
+      "md5": "b6679861fdce3bbab0c1fa51bb7f5077",
+      "rounds": 1,
+      "lastPage": 1,
+      "link": true,
+      "type": "eq"
+    },
     {  "id": "issue1015",
       "file": "pdfs/issue1015.pdf",
       "md5": "b61503d1b445742b665212866afb60e2",