coreaudio: handle unknown channels

Previously, unknown channels were just ignored, causing output channels mismatch. Unknown channels need to be handled by doing:

• Setup the renderer with the total number of channels given by kAudioDevicePropertyPreferredChannelLayout
• Realloc blocks, reorder them, and put 0s for unknown channels
• Change bytes<>ticks conversion factor (i_bytes_per_frame) to not mess with timings.

This can only happen on macOS when the user configured "Speakers" via the "Audio MIDI Setup" app.