This is a minor refactoring that moves a global bool to a boolean ivar in FlutterPlatformViewsController. The purpose of this variable is simply to avoid the overhead of trying to create backdrop filters if we've ever failed to create one in the past.
Given that this class will only ever have the one instance created and held by per engine with the same duration, and that most apps only ever have one engine, the performance win will be identical for most apps. For the few add-to-app cases with multiple engines either at once or over the course of an app's lifetime, the costs associated with firing up an engine are already a far bigger hit than those being saved by this bool.
Also migrates from C++ style namespace { ... } to Obj-C style static functions. These are entirely equivalent as both restrict symbols to the current translation unit.
This is a reland of https://github.com/flutter/engine/pull/56805, which was reverted as part of https://github.com/flutter/engine/pull/56817.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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