Use MALLOC consistently
Defer constructing object nodes
Use underlying type for state_game_id

This avoids the need for a header to declare it.
Fix Win32 OpenGL build
Rename struct _grs_* to struct grs_* for consistency
Make headers freestanding
Rename digi_max_channels -> digi_mixer_max_channels

digi_audio.cpp has its own digi_max_channels