[graphite] Make addition of runtime effects to ShaderCodeDictionary thread safe(r)

fUserDefinedCodeSnippets wasn't being guarded for thread safety and could've result in some problems.

Bug: b/238759147
Change-Id: Iaaf8a2c96e76a01488d257de30f2be440e7da371
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/816616
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
4 files changed