SDL: Use external midi sequencer software when arg with "-midicmd foo".

[Artanejp]

Configure for SDL/SDL2 with SDL_Mixer, loading and playing midi file(s) are very slowly and eats a lot of CPU time.

Because Mix_LoadMUS() tries to decode full track of a MIDI file to raw PCM data as default (maybe with some distros), this spend a lot of time.

But, at SDL_Mixer, available to use external command as a MIDI (or some formats) decoder, i.e. timidity.

So, I add a feature using external midi sequencer when running with ARG "-midicmd foo" or setting a environment variable; SIMUTRANS_MIDI_CMD.

[An-dz]

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