Add SkCodec methods for individual frames

Add a version of getFrameInfo that returns information about a single
frame, allowing a client to skip creating the entire vector.

Add getFrameCount, for determining the number of frames in the image.

Reimplement std::vector<FrameInfo> getFrameInfo with the new methods.

Updates to the test:
- getFrameInfo(size_t, FrameInfo*) fails before parsing
- Test both versions of getFrameInfo
- Recreate the codec between tests, to test parsing

Change-Id: I77c19087f2f8dcf2c536d80167b18ad1ca96ae94
Reviewed-on: https://skia-review.googlesource.com/13190
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Chris Blume <cblume@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
5 files changed