Don't try to readback from the GPU for small blur sigmas

This should revert Ganesh's small sigma handling behavior to back before:  https://skia-review.googlesource.com/c/skia/+/52400 (Reorganize blur filter to insert new implementation)

BUG=skia:7787
BUG=skia:7751
BUG=832838

Change-Id: I0ce6b888be534afb60336abf561e97741fa34684
Reviewed-on: https://skia-review.googlesource.com/121331
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Herb Derby <herb@google.com>
1 file changed