Roll Flutter Engine from 20034a8d62c4 to 4a603aaff32e (2 revisions) (#125004)

https://github.com/flutter/engine/compare/20034a8d62c4...4a603aaff32e

2023-04-17 magder@google.com Remove 'Mac mac_unopt' in favor of Linux (flutter/engine#41226)
2023-04-17 chinmaygarde@google.com [Impeller] Add debug ToString methods to render targets. (flutter/engine#41221)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/flutter-engine-flutter-autoroll
Please CC jacksongardner@google.com,rmistry@google.com,zra@google.com on the revert to ensure that a human
is aware of the problem.

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

bin/internal/engine.version

-20034a8d62c43ad3340a3ac09e36d01cf1d91d67
+4a603aaff32ec1bef709bcb9626a3b5b5ee3921b