Updates to images project.

Updates to images project.

Use the SkImageEncoder_Factory on all platforms. On Windows and Mac,
register the platform's image encoder as an option for
SkImageEncoder::Create. Also add more types that can be decoded.

Update comments for SkImageDecoder to be more accurate.

Add more types to SkImageEncoder::Type, and return the correct type of
encoder, if it exists.

Use a custom version of SkImageDecoder::Factory on Windows and Mac to
check the stream for registered decoders before defaulting to the platform's
version. Share code with the existing SkImageDecoder::Factory method.

Preparation for testing decoders and encoders:
BUG=https://code.google.com/p/skia/issues/detail?id=1241

Review URL: https://codereview.chromium.org/14298010

git-svn-id: http://skia.googlecode.com/svn/trunk@8730 2bbb7eff-a529-9590-31e7-b0007b416f81