Support 16-bit images in downsampler.

Support 16-bit images in downsampler.

The downsampler was not sampling correctly from 16-bit images, resulting in
using the RRG bytes (of the RRGGBB 16-bit pixel) for RGB in the 8-bit output.

BUG=583804

[ballbach, 2016-02-03 22:36:31]

Patchset #2 (id:20001) has been deleted

[ballbach, 2016-02-03 22:38:37]

ballbach@google.com changed reviewers:
+ thestig@chromium.org

[ballbach, 2016-02-03 22:38:38]

I would love to add an example PDF (there's one in the bug) to the corpus tests,
but I am not sure how to do that. The files in the corpus test directories
appear to come from third parties? Please advise on testing.

[Lei Zhang, 2016-02-03 23:13:01]

Description was changed from

==========
Support 16-bit images in downsampler.

The ddownsampler was not sampling correctly from 16-bit images, resulting in
using the RRG bytes (of the RRGGBB 16-bit pixel) for RGB in the 8-bit output.

See the bug here: https://code.google.com/p/chromium/issues/detail?id=583804

BUG=583804
==========

to

==========
Support 16-bit images in downsampler.

The downsampler was not sampling correctly from 16-bit images, resulting in
using the RRG bytes (of the RRGGBB 16-bit pixel) for RGB in the 8-bit output.

BUG=583804
==========

[Lei Zhang, 2016-02-03 23:13:56]

- Code looks good.
- I edited the CL description to fix a typo and deleted redundant info.
- I replied on the bug to discuss adding a test case.

[Lei Zhang, 2016-02-05 09:05:34]

lgtm, but I closed this CL and will land it as
https://codereview.chromium.org/1671833002/ instead.