render_pictures: add --mismatchPath flag

When set, it will only write out images that don't match expectations.

BUG=skia:1942
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/283123002

git-svn-id: http://skia.googlecode.com/svn/trunk@14748 2bbb7eff-a529-9590-31e7-b0007b416f81
14 files changed