Add currScanline() getter to SkCodec API

This is more correct than using nextScanline() for the
SkGifCodec scanline decoder (since we will get a strange
result in the interlaced case) and is necessary if we want
to add scanline decoding to SkIcoCodec.

This does not actually fix bugs or change behavior.

BUG=skia:

Review URL: https://codereview.chromium.org/1489163002
3 files changed