Infer sampler precision from pixel config

Adds a "samplerPrecision" method to GrGLSLCaps and updates
GrGLSLProgramBuilder to infer a sampler's precision based on its
config and visibility.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1846963004

Review URL: https://codereview.chromium.org/1846963004
9 files changed