Lazy SKP image decoding in DM.

@sugoi:
Early out to avoid some segfaults in SkImageDecoder_libjpeg.cpp.
I am just flailing here... things seem to work, but I have no idea why.
This prints out a lot:
    libjpeg error 85 <End Of Image> from output_raw_data [0 0]

@halcanary:
I'm skipping on ImageSrc for now.  Leon's refactoring that quite a lot.

This causes minor diffs for the GPU backend, given that we're now
going through the YUV path.  It also reduced peak RAM usage on
my desktop from 1.26GB to 1.08GB.

BUG=skia:

Review URL: https://codereview.chromium.org/1010983004
2 files changed