Improve performances with image masks (bug 857031)

- it's the second part of the fix for https://bugzilla.mozilla.org/show_bug.cgi?id=857031;
- some image masks can be used several times but at different positions;
- an image need to be pre-process before to be rendered:
  * rescale it;
  * use the fill color/pattern.
- the two operations above are time consuming so we can cache the generated canvas;
- the cache key is based on the current transform matrix (without the translation part)
  and the current fill color when it isn't a pattern.
- the rendering of the pdf in the above bug is really faster than without this patch.

test/pdfs/bug857031.pdf.link

@@ -0,0 +1,2 @@
+https://bug857031.bmoattachments.org/attachment.cgi?id=732270
+

test/test_manifest.json

@@ -6354,5 +6354,13 @@
          "value": "Hello PDF.js World"
         }
       }
+   },
+   {  "id": "bug857031",
+      "file": "pdfs/bug857031.pdf",
+      "md5": "f11ecd7f75675e0cafbc9880c1a586c7",
+      "rounds": 1,
+      "link": true,
+      "lastPage": 1,
+      "type": "eq"
    }
 ]