DM: support --config {f16,srgb}

Will need to follow up with enabling f16 and srgb configs.
They're still assert-y.

The GMs 'gamma' and 'gradients' look reassuringly good on f16 and srgb backends.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1711143002

Review URL: https://codereview.chromium.org/1711143002
3 files changed