add little-endian 16-bit support

Change-Id: Ic74a350e2accbf5b8696449bddc170ed818216f7
Reviewed-on: https://skia-review.googlesource.com/154001
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed