SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
Also removed minimum number of frames needed when decoding DPL2, and use
std::numeric_limits to clamp the samples when needed.
Clamping is still needed, but those samples are much rarer now and depend
on the game.
It seems to make no difference besides allowing lower latencies and more
stability on hardware OpenAL cards. Maybe the Wait() call waits for too
long, causing buffers underruns.
Before these changes each value of latency were actually 5ms, with a
minimum latency of ~10 ms. If it was set to 4 ms on the UI, the actual
latency was 10 + 5 * 4 = 30 ms.
Now 30 ms on the UI means 30 ms on the backend.
OpenALStream was querying the backend for AL_EXT_float32 support (which
suceeds), but AL_FORMAT_STEREO_FLOAT32 was defined incorrectly.
Also changes OpenALStream to query for AL_EXT_MCFORMATS (multichannel
support) rather than hard-coding that it doesn't work on macOS.