Properly set alpha type in webp decode.

Also use the newer setConfig function.

Add a test to confirm that we set the alpha type properly.

Add some images with alpha for testing. (These images are also beneficial for the compare_unpremul test, which was previously not meaningful on 100% opaque images.)

All of the added images are in the public domain. They were taken from https://developers.google.com/speed/webp/gallery2:

yellow_rose:
"Free Stock Photo in High Resolution - Yellow Rose 3 - Flowers"
Image Author: Jon Sullivan
This file is in the public domain.
http://www.public-domain-photos.com/free-stock-photos-4/flowers/yellow-rose-3.jpg

baby_tux:
"baby tux for my user page"
Image Author: Fizyplankton
This file is in the public domain.
http://www.minecraftwiki.net/images/8/85/Fizyplankton.png

NOTRY=true
R=halcanary@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/223893002

git-svn-id: http://skia.googlecode.com/svn/trunk@14054 2bbb7eff-a529-9590-31e7-b0007b416f81
8 files changed