TH08: MIDI BGM ignores the BGM volume setting

[nmlgc]

I expected that the setting would apply to both BGM modes.

[32th-System]

The function to set MIDI volume with the MIDI API the game uses seems to be midiOutSetVolume, however, when trying to call it, it simply returns MMSYSERR_NOTSUPPORTED, and the MIDI volume stays the same.

[nmlgc]

Touhou's MIDI code originates from Shuusou Gyoku, which had the same issue. The best solution there was intercepting every CC #7 message and factoring the BGM volume setting into its parameter; using the semantically correct Universal Realtime Master Volume message would actually be more of a hack.