De-SkScalar SkRect

Change SkRect over to using floats instead of SkScalar.
Introduced sk_float_midpoint to handle some of the
calculations.

TODO:
* rename asScalar -> asFloats possibly? Separate CL.
* what to do with all the scalar string stuff in dump.

Change-Id: I8416f17654d9e73ff094d4c5a42ac2669389ceda
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/714063
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed