Add dest type hint to SkShader::ContextRec

Let SkBlitter decide which dst type is optimal (PMColor vs PM4f), and
pass that info to shaders.

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1724503002

Review URL: https://codereview.chromium.org/1724503002
7 files changed