Use case

Flutter GPU can be compiled into some toolchains when impeller_supports_rendering is false, even though creating a usable GpuContext already requires Impeller at runtime. This makes implementation code carry redundant !IMPELLER_SUPPORTS_RENDERING paths and can obscure which native symbols and files are expected to exist in non-rendering builds.

This came up while adding GpuSurface, where the surface implementation depends on DlImageImpeller and only makes sense when Impeller rendering support is compiled in.

Proposal

Split the Flutter GPU native build so non-rendering configurations compile only the minimal code needed to report the existing clean GpuContext initialization error, while the full Flutter GPU implementation is compiled only when impeller_supports_rendering is true.

As part of this cleanup, remove redundant non-rendering runtime branches from implementation files and verify that Dart @Native imports and symbol resolution still behave correctly in configurations where Flutter GPU is unavailable.