Minor fixes/adjustments to the GLFW shell (#8990)

- Makes json_method_codec.cc compatible with the last stable RapidJSON release.
- Allows removing the GTK dependency with a compile flag.
- Fixes a missing break in a switch flagged by some toolchains.
https://dart.googlesource.com/external/github.com/flutter/engine/+/75678602b49c67bd103f0ec16b676f8a975331bd
diff --git a/commits.json b/commits.json
index 21c9902..c0efc66 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"effc214730d8943c9fb37fd0bd9056839b4363be",
+  "external/github.com/flutter/engine":"75678602b49c67bd103f0ec16b676f8a975331bd",
   "external/github.com/flutter/flutter":"c0125cb44a74dae6fd2cc3888546ebd7e30ce227",
   "sdk":"e9b5c9ce8b57c3073d734e916e0c1a2c0cd63751"
 }
\ No newline at end of file