WAV importer doesn't support Loop Modes

[WiggleWizard]

Godot version:
3.2.1.stable.official
Checked 3.2 and master, both contain the issue.

OS/device including version:
Not OS specific.

Issue description:
I was kinda hoping that audio would have the possibility to play in reverse. However there's no exposed GDScript way to achieve that. So as a workaround I attempted to move the playhead around beyond the loop marker and set the AudioStreamSample Loop Mode to "Ping-Pong" to hopefully achieve the same effect. I found that didn't work, I then realized that looping in an exported game will only loop forwards but in the editor it loops in the way I select.

This lead me to the import properties of the WAV file in question and found there's no supporting "Loop Mode" option in the import dialog to support the AudioStreamSample resource mapping. Naturally this results in the developer being artificially limited to Disabled and Forward loop modes.

Steps to reproduce:

1. Import a WAV file with loop on.
2. Put loop mode in resulting AudioStreamSample to "Ping-Pong".
3. Re-select the AudioStreamSample. Watch "Loop Mode" get reset to "Forward".

Code area
https://github.com/godotengine/godot/blob/3.2/editor/import/resource_importer_wav.cpp#L88. Either the loop property needs to be changed to loop_mode with the appropriate enum, or a loop_mode property should be added.

[akien-mga]

Loop mode is a property of WAV files and is thudnhardcoded in the file.

If you want to override it, edit the WAV file's metadata, or save the imported version as a .tres file and edit properties.

[WiggleWizard]

It should be clear that you need to mod the WAV metadata or converting through importing as a .tres file imho. Perhaps that's the work that needs to be done?

Personally it would be really great if the general agreement was to just add the ability to modify the loop_mode upon import. It works fine in the editor, don't see why it wouldn't work fine in an export.

[KoBeWi]

Seems implemented in 3.5