Roll Flutter Engine from b40042ebb95b to 3b0469b0c718 (2 revisions) (#136922)

https://github.com/flutter/engine/compare/b40042ebb95b...3b0469b0c718

2023-10-19 jason-simmons@users.noreply.github.com [Impeller] Use a weak pointer factory in GPUSurfaceGLImpeller that supports the raster thread merger (flutter/engine#47081)
2023-10-19 skia-flutter-autoroll@skia.org Roll Dart SDK from ea86103e06b1 to f71ddbe0a500 (1 revision) (flutter/engine#47124)

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 jimgraham@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://issues.skia.org/issues/new?component=1389291&template=1850622

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

bin/internal/engine.version

-b40042ebb95b064e336d20a0bc99807901be0648
+3b0469b0c718cdd69570c0830743460d5f159200