[svg] Cleanup: use FP alpha for opacity

Update general opacity and stop-opacity to use float alpha.

Change-Id: I496b2d41d77c0123ea2a2e1f8f8e4b1377a98abc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/355627
Auto-Submit: Florin Malita <fmalita@chromium.org>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
7 files changed