Update Music.h for SDL_Mixer 2.8.1

[DanRStevens]

In Music.h there is a forward declaration:

typedef struct _Mix_Music Mix_Music;

The name _Mix_Music is a reserved identifier, since it starts with an underscore followed by an uppercase letter. This was reported to the SDL_Mixer project, which updated the name:

typedef struct Mix_Music Mix_Music;

The Arch Linux package has already updated to the new version. Unfortunately this causes the NAS2D build to break due to the incompatible name change.

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Potentially we can revert to including the full SDL_mixer.h header file, rather than using a forward declare. We can possibly switch back to a forward declare once all supported builds have stabilized on the new version.

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The problem of creating a forward declaration for an unknown struct is interesting.

https://stackoverflow.com/questions/7256436/forward-declarations-of-unnamed-struct

The idea presented there of created a new struct that derives from the old one was interesting. Unfortunately that's not possible when the base struct type is an incomplete type.

// h
struct MusicHandle;

// cpp
struct MusicHandle : public _Mix_Music {};

error: invalid use of incomplete type ‘struct _Mix_Music’

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Maybe we shouldn't have Music as a standalone class, but rather something created or managed by MixerSDL. This might be similar to the idea of having RendererOpenGL own and manage Texture objects, rather than embedding that functionality in Image.

Related:

• Issue Image vs Texture confusion #968