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

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!
Committed: https://code.google.com/p/skia/source/detail?r=3975

[epoger, 2012-05-16 17:39:58]

I'm gonna go ahead and commit this... if there are objections, I will roll it
back out or make changes in a future CL.  I hope that's OK.

[TomH, 2012-05-16 17:44:00]

Ex post facto LGTM on the skdiff_main change.

Why all deletions of the pngs?

[epoger, 2012-05-16 17:49:22]

On 2012/05/16 17:44:00, TomH wrote:
> Why all deletions of the pngs?

Because they aren't expected to be generated anymore.  (Due to the skdiff
change)