QR thumbnail previews creation should be aware of the device pixel ratio.

src/Posting/QR.coffee

@@ -601,7 +601,7 @@ QR =
         # Resized pictures through canvases look like ass,
         # so we generate thumbnails `s` times bigger then expected
         # to avoid crappy resized quality.
-        s = 90*2
+        s = 90 * 2 * window.devicePixelRatio
         s *= 3 if @file.type is 'image/gif' # let them animate
         {height, width} = img
         if height < s or width < s