DM:
Add a flag to use SkCodec instead of SkImageDecoder.
SkCodec:
Base class for codecs, allowing creation from an SkStream or an SkData.
An SkCodec, on creation, knows properties of the data like its width and height. Further calls can be used to generate the image.
TODO: Add scanline iterator
SkPngCodec:
New decoder for png. Wraps libpng. The code has been repurposed from SkImageDecoder_libpng.
TODO: Handle other destination colortypes
TODO: Substitute the transpose color
TODO: Allow silencing warnings
TODO: Use RGB instead of filler?
TODO: sRGB
SkSwizzler:
Simplified version of SkScaledSampler. Unlike the sampler, this object does no sampling.
TODO: Implement other swizzles.
BUG=skia:3257
Review URL: https://codereview.chromium.org/930283002
