iOS: isGpuDisabled set before engine start is silently clobbered by createShell #190835

FlutterEngine.isGpuDisabled is a public read/write property, but a value assigned to it before the engine runs is silently clobbered before it ever reaches the shell.

-[FlutterEngine createShell:libraryURI:initialRoute:] samples the current lifecycle state by assigning to the property, immediately before constructing the shell:

The value is then passed to Shell::Create as is_gpu_disabled. Because the sampling is an unconditional assignment to a public property, it also destroys whatever the caller set beforehand. An embedder that does:

engine.isGpuDisabled = YES;
[engine run];

silently gets the GPU enabled, with no warnings whatsoever.

The sampling itself is necessary and shouldn't be deleted. Lifecycle notifications only report transitions, so an engine created while the application or scene is already backgrounded never receives one and would otherwise come up believing the GPU is available.

The problem is more with where we do this work and the fact that it silently clobbers the state with no warning.

Why is this a problem for Embedder API migration?

FlutterEngineRun would naturally accept initial GPU availability as a project argument, if we add it. Passing that property straight through would resurrect exactly the stale value that createShell: currently goes out of its way clobber.