skdiff: only write out diff images that are included in index.html

Not only are those files unnecessary, in some different-image-size cases, their contents are nondeterministic!
Review URL: https://codereview.appspot.com/6208073

git-svn-id: http://skia.googlecode.com/svn/trunk@3975 2bbb7eff-a529-9590-31e7-b0007b416f81
8 files changed