Move sk_float_nextlog2 to WangsFormula.h and cleanup

Add comments and remove the arithmetic shift rights from
the code. Rename to nextlog2. Move tests.

Change-Id: I716541c7e72caf61c5db43962ef21be84e82adc1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/721913
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
4 files changed