Set the minimum required Metal version

[emezeske]

Apple's documentation says that the language version will be set to the most recent version if it's not specified, but this is flat out false in some environments. On Apple Silicon I believe it is true, but I recently ported my app to run as a universal binary, and I have an Intel Mac where in some contexts, Filament's shader compilation fails with errors like:

Compilation failed:

program_source:113:41: error: 'id' attribute requires Metal language standard macos-metal2.0 or higher
constant FrameUniforms* frameUniforms [[id(0)]];
                                        ^~~~~

My app works when running as a standalone executable, but this issue comes up when running as a plugin (dynamically loaded object) inside another application. It appears that the host application can somehow cause the Metal API to assume a different Metal version than the latest.

Setting this explicitly to the minimum version that Filament requires fixes the problem. It also seems like a good idea, because it makes the version requirement explicit -- if a new Filament version is going to use newer Metal API features, it will be clear to everyone what version of Metal is being targeted.