Implement postfix ++/--, and use the correct 1 for floats

Change-Id: I0e6e8205998f3058cb59e279704e23989bccc66b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214192
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed