Use the ManagedBackendTexture helper in fiddle_main

Change-Id: I84dd14cc3328bf2f9beb0c0ac17a4f98fb62baba
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/273017
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed