Add imagemasksubset GM

(GM for non-N32 SkImage subsetting)

BUG=skia:5687
R=bsalomon@google.com,robertphillips@google.com,reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2286733002

Review-Url: https://codereview.chromium.org/2286733002
35 files changed