GMCChannel support for loops

[avanwinkle]

This PR introduces support for looping from MPF sound_player triggers and MPFSoundAsset resources. Manually tested with sound files, sound asset resources, and AudioStreamRandomizer resources.

Addresses issue #15

Summary

The Godot classes AudioStreamOGG and AudioStreamWAV support a native loop property which will endlessly loop the sound playback, while AudioStreamMPF supports a loop_mode property which loops playback in a variety of ways. Unfortunately:

• these properties abstracted from the AudioStreamPlayer (or GMCChannel) which prevents any count of looping
• AudioStreamRandomizer and other stream types do not have any native looping property

The Fix

This PR adds loop-management logic to GMCChannel to manually loop an audio stream by hooking into the finished signal and calling play() again. For situations where a specific number of loops are requested, metadata is attached to the stream to count the number of loops.

When the stream playback finishes, the channel will check the loop state and either re-play the stream or stop itself.

Note: For situations where the audio stream is a native Godot type that supports looping and the looping is infinite, the native looping property is set and the channel does not manually manage looping.