DM: fix -w/-r problems stemming from PM/UPM conversions.

DM: fix -w/-r problems stemming from PM/UPM conversions.

DM will now read and write a custom image format, which begins first with a
normal PNG, but also includes the bitmap's raw pixels.  Most tools (file
browser, Preview, Chrome, Skia image decoder) will read the file as PNG.  DM
skips the encoded PNG and reads the raw pixels instead.

This scheme allows perfect pixel comparisons that never go through PM -> UPM or
UPM -> PM conversions, as long as you compare on a machine with the same native
pixel format as the machine which generated the images.  Since we've only ever
used this to do same-machine comparisons, that's more than good enough for now.
We could convert as needed to a standardized PM pixel format if we find we want
these files to be portable.

DM -w output now does increase to ~1.3G.  If that turns out to be annoyingly
big, I'm sure I can come up with a simple pixelwise RLE instead of writing
uncompressed pixels.

BUG=skia:

Committed: http://code.google.com/p/skia/source/detail?r=13638

[reed1, 2014-03-03 14:05:00]

lgtm

age-old-question: does WebP support native premul, so we could use it for
perfect in/out?

[mtklein, 2014-03-03 14:41:17]

On 2014/03/03 14:05:00, reed1 wrote:
> lgtm
> 
> age-old-question: does WebP support native premul, so we could use it for
> perfect in/out?

Ah yes, I did investigate this.  WEBP does not support native premul.  The most
familiar format that I could find that supports native premul is TIFF.  TIFF
supports everything.  But it's not quite normal enough for me to be happy with:
Chrome doesn't support TIFF.

[mtklein, 2014-03-03 14:58:13]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-03-03 14:58:22]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/mtklein@chromium.org/185343002/20001

[commit-bot: I haz the power, 2014-03-03 15:43:57]

Change committed as 13638