Roll Skia from 271b2b6d5aaa to 975eb1250431 (2 revisions) (#43712)

https://skia.googlesource.com/skia.git/+log/271b2b6d5aaa..975eb1250431

2023-07-15 [email protected] Fix error on tree about unused backbuffer variable.
2023-07-14 [email protected] SkWuffsCodec: allow Wuffs versions >= v0.3.1

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-flutter-autoroll
Please CC [email protected],[email protected],[email protected],[email protected] on the revert to ensure that a human
is aware of the problem.

To file a bug in Skia: https://bugs.chromium.org/p/skia/issues/entry
To file a bug in Flutter: https://github.com/flutter/flutter/issues/new/choose

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

Author: skia-flutter-autoroll

DEPS

@@ -18,7 +18,7 @@ vars = {
   'llvm_git': 'https://llvm.googlesource.com',
   # OCMock is for testing only so there is no google clone
   'ocmock_git': 'https://github.com/erikdoe/ocmock.git',
-  'skia_revision': '271b2b6d5aaa62e39eba9b526292782842236186',
+  'skia_revision': '975eb12504312c00cddad640dcbc01ce41a71dd7',
 
   # WARNING: DO NOT EDIT canvaskit_cipd_instance MANUALLY
   # See `lib/web_ui/README.md` for how to roll CanvasKit to a new version.

ci/licenses_golden/licenses_skia

@@ -1,4 +1,4 @@
-Signature: 5cc56e9dd46382c8e24c6b3a20b7b3b9
+Signature: c25e6a306f4ed0359e6b0ed93e35dbfb
 
 ====================================================================================================
 LIBRARY: etc1