Use interpolating blur for sigma < 2

Reimplemnet the original interpolating code. Moving from the
integer blur code to this code is controled by the flag:
SK_LEGACY_SUPPORT_INTEGER_SMALL_RADII

BUG=chromium:759273

Change-Id: I17205d704a0cae68a8a027c6bb235f81b5c62e24
Reviewed-on: https://skia-review.googlesource.com/41082
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed