Add detection of image load/store to caps

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4861

Change-Id: Ibf70cce9f61553e830b62339551c1541dc054e8b
Reviewed-on: https://skia-review.googlesource.com/4861
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
5 files changed