Add a GL cap for instanced rendering to floating point

We have observed on OS X that instanced draws don't always behave
correctly when the framebuffer is floating point.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2095183002

Review-Url: https://codereview.chromium.org/2095183002
3 files changed