Make skdiff always generate white diffs alongside actual pixel diffs

Make skdiff always generate white diffs alongside actual pixel diffs
(and some associated improvement to the output)
Committed: https://code.google.com/p/skia/source/detail?r=3138

[epoger, 2012-02-02 20:01:04]

(If you'd like to see sample output with these modifications, come on over)

[epoger, 2012-02-02 20:04:29]

On 2012/02/02 20:01:04, epoger wrote:
> (If you'd like to see sample output with these modifications, come on over)

P.S. I think we should consider re-ordering the columns in the output as
follows:
white | diffs | base | comparison

(it will be easier to visually compare base and comparison images if they are
side-by-side)

[TomH, 2012-02-02 20:38:25]

LGTM.

Passing in diffFunction to computeDiff() is now obsolete.

I don't have a strong opinion about re-ordering the columns, just make sure you
look at it yourself first to make sure your intuition is reasonable.

[epoger, 2012-02-02 20:51:32]

On 2012/02/02 20:38:25, TomH wrote:
> LGTM.
> 
> Passing in diffFunction to computeDiff() is now obsolete.

I left that alone, just in case we wish to add a different diffFunction in the
future.
 
> I don't have a strong opinion about re-ordering the columns, just make sure
you
> look at it yourself first to make sure your intuition is reasonable.

I reordered the columns.  It definitely helps (IMHO)