Support decoding Gray to A8 in PNG.

Support decoding Gray to A8 in PNG.

Move the code which sets the filler and forces gray to rgb after
we get the config, so we can skip them if the caller wants A8.

Call set_gray_to_rgb consistently for both normal decode and subset
decode.

In PNG, prevent subset decodes from alternating configs, which
would otherwise fail.

Use SK_RESTRICT for pointers in getBitmapConfig.

Ref the SkStream input to buildTileIndex, so it will not be destroyed
before calling decodeSubset.

Convert some fields to match Skia style.

Builds on https://codereview.chromium.org/18083026/, which has not yet been checked in.

R=reed@google.com

Committed: https://code.google.com/p/skia/source/detail?r=10162

[scroggo, 2013-07-16 16:09:05]

Oops, removing Mike's personal email.

This change still has a bug, as it allows tile decode to switch the preferred
config between subset decodes, which fails.

[scroggo, 2013-07-16 21:09:12]

Latest patch fixes the aforementioned bug.

[reed1, 2013-07-18 14:21:04]

lgtm

[scroggo, 2013-07-18 20:03:21]

Committed patchset #3 manually as r10162 (presubmit successful).