If ZMusic is to act like an external library it may not call delete on external objects because there is no guarantee that they use the same allocator. Deletion must be done as a virtual function to ensure that the correct operator delete gets called, which, unlike the actual destructor is not virtual itself.
Since this gets used by both the sound backend and the music code it needs to be in a place accessible to both.
# Conflicts:
# src/CMakeLists.txt
# Conflicts:
# libraries/zmusic/decoder/mpg123_decoder.cpp
# libraries/zmusic/decoder/sndfile_decoder.cpp
# src/CMakeLists.txt
This was getting a bit unwieldy. The include path setup is not perfect yet, that's work for later.
(It's about time we're getting C++20 with modules so that this include path madness can be put to an end.)
