Default feature level in the C API as well

[Kangz]

Prior to this it would have been allowed to call wgpuInstanceRequestAdapter(instance, nullptr) to get any adapter that the instance chooses. Can't we say that it defaults to core like the JS API?

Originally posted by @Kangz in #399 (comment)

[kainino0x]

See #293 for past discussion, I'll close that one in favor of this more specific one.

[kainino0x]

Prior to this it would have been allowed to call wgpuInstanceRequestAdapter(instance, nullptr) to get any adapter that the instance chooses. Can't we say that it defaults to core like the JS API?

This is a good point, I forgot this means you have to pass a whole descriptor. I am fine with matching the JS API.

[kainino0x]

Nov 21 meeting:

• KN: We previously discussed making Compatibility the default. Corentin prefers to default to Core to match JS. Kind of nice to make it required, but then the whole WGPURequestAdapterOptions struct becomes required. OTOH, non-breaking to add default later.
• CF: wgpu doesn't really have compatibility mode but effectively defaults to Core. Would prefer to make this required, but can see the argument for defaulting to core. Have at times regretted the default to core in wgpu, but WebGPU is already more limited in its default device.
• Default to core.