Don't downscale by steps images which will end up with small dimensions (bug 1445735)

src/display/canvas.js

@@ -1188,6 +1188,16 @@ class CanvasGraphics {
       1
     );
 
+    if (width / widthScale <= 5 && height / heightScale <= 5) {
+      // The final image is very small, hence having a poor quality
+      // when rescaling isn't really a problem.
+      return {
+        img,
+        paintWidth: width,
+        paintHeight: height,
+      };
+    }
+
     let paintWidth = width,
       paintHeight = height;
     let tmpCanvasId = "prescale1";

test/pdfs/bug1445735.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=8958936

test/test_manifest.json

@@ -7713,5 +7713,13 @@
         "value": ["Three", "Five"]
       }
     }
- }
+   },
+   {
+      "id": "bug1445735",
+      "file": "pdfs/bug1445735.pdf",
+      "md5": "a5470d82c1d23eac1fef2fa37f95e980",
+      "rounds": 1,
+      "link": true,
+      "type": "eq"
+   }
 ]