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.
Migrates the state to be instance-based as opposed to being a flat
namespace. This keeps behavior localized to its own instantiable unit
(and forces uses of the class to also be localized, lest they cart around
an instance all over the place).
The main problem was that the volume of the mixer wasn't savestated.
The volume is typically 0 at the beginning of a game, so loading a
savestate at the beginning of a game would lead to silent DTK audio.
I also added savestating to StreamADPCM.cpp.
