Fix about bad duration set and add sample with Opus Codec implementation

[ChristopheI]

SDL2AudioSource:

• allow to specify frame size in constructor and use it to dequeue sample from output device.
• Fix call to OnAudioSourceEncodedSample with invalid duration.

SDL2Helper:

• add GetAudioRecordingDevice (by index) and GetAudioPlaybackDevice (by index) methods
• GetAudioSpec now accept a new parameter: sample

CheckCodec example:

• Implements Opus codec using Concentus C# implementation
• Permits to test available Codecs (Encoding/Decoding): PCMU, PCMA, G722, G729, Opus