speedup exp() in gaussian colorfilter

~20% faster
differs in low 1 from prev impl

Bug: skia:
Change-Id: If7aa7f5523f15a5424ebf5a09313c5e1d43e1a1f
Reviewed-on: https://skia-review.googlesource.com/15105
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
1 file changed