SkMatrix44 clarifications and clean-ups

Fixed row/col major bug and added comments to the
header.

Made SkMatrix::I() thread safe using constexpr.

Added a test set3x3RowMajorf().

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2098583002

Review-Url: https://codereview.chromium.org/2098583002
6 files changed