Remove singleton of Transport, Destination, Master, Listener, Draw and context

These singletons call getContext() when page is initializing. Therefore, the error from getContext will not be able to catch and such will caused whole webpage broken.

Add polyfill for AudioParam.cancelAndHoldAtTime

Add polyfill for constant-source-node

Add polyfill for promise OfflineContext.startRendering

Add polyfill copyToChannel and copyFromChannel for AudioBuffer

enable createScriptProcessor

make Tone.Param._param.getValueAtTime bind to Tone.Param's getValueAtTime

Add polyfill StereoPannerNode, comment out set panner channelCount in CrossFade and Panner, because it will cause safari slient with stereoNode polyfill

If the useragent is Safari, then use original offline context instead of stdOfflineCtx, otherwise, there will be Attempted to assign to readonly property. Error throw fromt ToneConstantSource constructor, I find this bug in [email protected], macOS Mojave