Update SkVx to_half to support denormals,overflow, and RTNE

Change-Id: I3155be97ceaeae17c90bb5d9c3e25f811ff10630
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/798976
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
10 files changed