refactor approx_{log2,pow2,powf}

    - Move to SkJumper_vectors.h
    - Fold the -127 and +2.774485010.
    - approx_powf(F,F) instead of approx_powf(F,float) for consistency.
    - A little layout reformatting.

Change-Id: If9cb3d62a097cb6ecf89f157a1dde672c1516371
Reviewed-on: https://skia-review.googlesource.com/13865
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed