GLES2 is not designed to be a drop-in replacement for the GLES3 backend,
so the fallback mode has to be used knowingly. It *can* make sense for
simple projects which make sure to handle the differences between both
rendering backends, but most users should stick to one supported backend.
By making it opt-in, we can now use this parameter to define whether to
export ETC textures to Android and iOS when using GLES3 + Fallback.
When using GLES3 without Fallback on Android, set the proper min GLES
version in the AndroidManifest.
Also made the option boolean and renamed it for clarity and to avoid
conflict with the previous String option (which would always evaluate as
"true" otherwise).
Fixes#26569.
b0f782a0e3
