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

Review URL: https://codereview.chromium.org/19185006

git-svn-id: http://skia.googlecode.com/svn/trunk@10162 2bbb7eff-a529-9590-31e7-b0007b416f81