Add "sample locations" feature to GrProcessor

Adds a "sample locations" feature to GrProcessor. When enabled, this
allows a processor to know inside the shader where all the samples are
located. Also adds various infastructure to query, cache, and identify
multisample data.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1717393002

Review URL: https://codereview.chromium.org/1717393002
17 files changed