Add sk_float_midpoint

Calculate the midpoint of a and b in such a way that
it does not overflow or underflow.

Change-Id: I739c91ddf65b930a4c1689a1713480ca0820b71e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/714064
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed