Fixes for decoding to A8.

Fixes for decoding to A8.

src/images/SkImageDecoder_libpng.cpp:
A8 images are not opaque, so do not set the opaque flag.
This fixes a bug where copyTo does not work as expected (when
copying an A8 decoded image to ARGB_8888), leading to
a bitmap hash that does not represent the image correctly
(in skimage).

tools/skimage_main.cpp:
In write_bitmap, which is creating the image for visual comparison,
copy A8 to 8888, since A8 cannot be encoded.

In the section that tests reencoding, do not test reencoding A8,
which is known to not work.

R=mtklein@google.com, djsollen@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/25726004

git-svn-id: http://skia.googlecode.com/svn/trunk@11589 2bbb7eff-a529-9590-31e7-b0007b416f81