Roll Flutter Engine from 2dcfb6cb7e49 to 394554bd1630 (4 revisions) (#132918)

https://github.com/flutter/engine/compare/2dcfb6cb7e49...394554bd1630

2023-08-20 chinmaygarde@google.com [Impeller] Document threading configuration with Vulkan. (flutter/engine#44874)
2023-08-20 skia-flutter-autoroll@skia.org Roll Fuchsia Mac SDK from onEHyyETXAMieDyfP... to fiGH0e7wu87NiNtTr... (flutter/engine#44875)
2023-08-20 skia-flutter-autoroll@skia.org Roll Dart SDK from a4908f67b63e to 632c8cccd30c (2 revisions) (flutter/engine#44872)
2023-08-20 skia-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from pSqQ556xmZp7S4np5... to iQbvLoFBbhU8pkkqw... (flutter/engine#44878)

Also rolling transitive DEPS:
  fuchsia/sdk/core/linux-amd64 from pSqQ556xmZp7 to iQbvLoFBbhU8
  fuchsia/sdk/core/mac-amd64 from onEHyyETXAMi to fiGH0e7wu87N

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 jsimmons@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

-2dcfb6cb7e494d9cfa316dcddbf94e9109bc01cb
+394554bd1630be06983c8e367d9efc4cd2195040

bin/internal/fuchsia-linux.version

-pSqQ556xmZp7S4np5xyQXWu-Ul-5qoxDxu8WXLe2_xIC
+iQbvLoFBbhU8pkkqwv_aYZJl-o6VuBXtm09qR63mFIEC

bin/internal/fuchsia-mac.version

-onEHyyETXAMieDyfP8hzVi3zqabh27iPDE9waYIkPc0C
+fiGH0e7wu87NiNtTrdmMJXDKjhuYP--6b-UzpOT55dIC