[M72 cherry-pick] Fix truncation of sigma parameter in blur-mask-filtered mask key

This would allow blur-masks alias one another.

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Bug: 912595
Bug: 878195
Change-Id: I44e86e893a505cf8abd991975a025cda52011db6
Reviewed-on: https://skia-review.googlesource.com/c/182665
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Robert Phillips <robertphillips@google.com>