DM: conserve memory when using --skps

 - limit to maximum 1000x1000 viewport like in bench_pictures and nanobench
 - use lazy bitmap decoding when reading the pictures from disk

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/448263002
3 files changed