Safari 12.1.2 and macOS 10.14.6: Fixed stereo

[nikita-kun]

player.context.channelCount always has a value of 2.
maxChannelCount is 0.

AudioDestinationNode {maxChannelCount: 0, context: webkitAudioContext, numberOfInputs: 1, numberOfOutputs: 0, channelCount: 2, …}