audio: Don't ever use libsamplerate in the SDL_AudioCVT codepath. It causes audio pops if you're converting in chunks (and needs to allocate/initialize/free on each convert). We'll either adjust this interface when we break ABI for 2.1 to make this usable, or publish the SDL_AudioStream API for those that want a streaming solution. In the meantime, the "simple" resampler produces "good enough" audio without pops and doesn't have to be initialized, so that'll do for now on the SDL_AudioCVT interface.