Add Options to SkDecodingImageGenerator, simplify API.

Add Options to SkDecodingImageGenerator, simplify API.

Motivation: We want to remove redundant classes from Skia.  To
that end we want to remove SkImageRef and its subclasses and
replace their uses with SkDiscardablePixelRef +
SkDecodingImageGenerator.  Since Android uses SkImageRef, we need
to make sure that SkDecodingImageGenerator allows all of the
settings that Android exposes in BitmapFactory.Options.

To that end, we have created an Options struct for the
SkDecodingImageGenerator which lets the client of the generator set
sample size, dithering, and bitmap config.

We have made the SkDecodingImageGenerator constructor private
and replaced the SkDecodingImageGenerator::Install functions
with a SkDecodingImageGenerator::Create functions (one for
SkData and one for SkStream) which now take a
SkDecodingImageGenerator::Options struct.

Also added a ImageDecoderOptions test which loops through a list
of sets of options and tries them on a set of 5 small encoded
images.

Also updated several users of SkDecodingImageGenerator::Install to
follow new call signature - gm/factory.cpp, LazyDecodeBitmap.cpp,
and PictureTest.cpp, CachedDecodingPixelRefTest.cpp.

We also added a new ImprovedBitmapFactory Test which simulates the
exact function that Android will need to modify to use this,
installPixelRef() in BitmapFactory.

R=reed@google.com, scroggo@google.com

Committed: https://code.google.com/p/skia/source/detail?r=12744

Review URL: https://codereview.chromium.org/93703004

git-svn-id: http://skia.googlecode.com/svn/trunk@12855 2bbb7eff-a529-9590-31e7-b0007b416f81