don't use MASK_FORMAT_UNKNOWN to represent scaler context state

MASK_FORMAT_UNKNOWN was shared state between glyphs, and
SkScalerContext making hard to reason about.

Change-Id: Ifb70bcc288bc46905de5f2c32891c940a0b2a11f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334157
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
9 files changed