Roll Flutter Engine from d4b6b7ec8e48 to 021a5ff5eae6 (5 revisions) (#141651)

https://github.com/flutter/engine/compare/d4b6b7ec8e48...021a5ff5eae6

2024-01-16 ditman@gmail.com [web] Leave blob URLs untouched in TT policy. (flutter/engine#49782)
2024-01-16 jason-simmons@users.noreply.github.com [Impeller] Fix a race between SwapchainImplVK::Present and WaitForFence (flutter/engine#49777)
2024-01-16 737941+loic-sharma@users.noreply.github.com Reland "[Windows] Move to FlutterCompositor for rendering" (flutter/engine#49726)
2024-01-16 jason-simmons@users.noreply.github.com [Impeller] Add an API for sampling strictly within the bounds of the source rect in DrawImageRect (flutter/engine#49696)
2024-01-16 jonahwilliams@google.com [Impeller] Encode directly to command buffer for Vulkan. (flutter/engine#49780)

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 jonahwilliams@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

-d4b6b7ec8e48eddcdc45c2162085ba3430733042
+021a5ff5eae64f986acdf25932ae57c8492afd44