Improving the music playback

For the past week I've been working on a number of improvements to the EMI sound system in ResidualVM. My change set in PR #972 improves especially the music playback in the PC version of the game.

Firstly, the music playback now starts at the correct position of the track, and the correct sub-section of the track is looped. This is because the music player now respects the cue point data which is stored in files with the file suffix ".jmm" alongside the actual MP3 music files.

The .jmm files may contain 'start', 'jump' and 'end' cues. The 'start' cue specifies a position in milliseconds where the music track should start playback. The 'jump' cue specifies two positions on the track. Once the first position is reached, the playback will jump to the second position. This cue is typically used to loop a sub-section of the music track. If the JMM file does not specify a 'jump' cue, the track does not loop (an example of this is the chapter screen music). The 'end' cue seems to always have a value of -1 in EMI, so I chose not to implement handling for it at this time.

Secondly, music syncing is now implemented. In some parts of the game, when the music track changes the new track should start from the same position where the previous track ended. Typically such synced tracks are slight variations of the same composition. For example, when Guybrush is close to the fountain in Melee town (set mel), the music changes to a version of the Melee town music with a sound of flowing water mixed in.

The information whether a music track should be synced with another track or not is stored in a file called FullMonkeyMap.imt. This file maps music track id numbers (which are referenced from the Lua scripts) to sound files, and specifies some parameters for each track. One of these parameters is called "sync", and has an integer value associated with it. A value of 0 seems to indicate that the track should never be synced. Otherwise, if the sync value matches the sync value of the track that was playing previously, the track will start from the same position where the previous track was at the time of transition.

Another improvement to music playback is when the music track changes, the music now transitions with a linear cross-fade, instead of instantly cutting to the new track. This makes the transitions sound less abrupt, and matches the behavior of the original game.