Add first pixel diffing test.

This is just a simple example of font scaling, so we have some
bits to diff against. I've added a /pixel directory to separate
these from the embeddertest resources.

BUG=https://code.google.com/p/pdfium/issues/detail?id=62
R=jam@chromium.org, thestig@chromium.org

Review URL: https://codereview.chromium.org/904983002