Roll Flutter Engine from a9a6f598f007 to 55670b71eb00 (1 revision) (#147091)

https://github.com/flutter/engine/compare/a9a6f598f007...55670b71eb00

2024-04-19 jonahwilliams@google.com [Impeller] organize texture shaders / delete blend.frag + external_texture_fill (flutter/engine#52137)

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://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
diff --git a/bin/internal/engine.version b/bin/internal/engine.version
index 35792e1..5a08923 100644
--- a/bin/internal/engine.version
+++ b/bin/internal/engine.version
@@ -1 +1 @@
-a9a6f598f0072e7a5712372efb87953f26f0bf34
+55670b71eb00fbe80601b7227f68c1df4cae827b