Added a Dolby Pro Logic II (DPL2) decoder in the OpenAL backend. DPL2 audio is decoded to 5.1. Code adapted from ffdshow. Added an option in the DSP settings to disable the DPL2 decoder in case Dolphin incorrectly detects a 5.1 audio system. Updated the OpenAL files to OpenAL Soft 1.15.1 in the Windows build. Fixes issue 3023.