Fix colorType/alphaType checks in SkCodec

Make getPixels() and startScanlineDecode() behave
consistently.

Require that kGray8 decodes are opaque.

Assert that creating the swizzler succeeds.

BUG=skia:4203
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1695473002

Committed: https://skia.googlesource.com/skia/+/c7578b6cdd03b61f076ffc7956efd952d6c301c0

Review URL: https://codereview.chromium.org/1695473002
10 files changed