[vlc-devel] Add equalizer API to libvlc.

[Mark Lee]

This new patch hopefully addresses all previous review comments.

Significant changes in this patch compared with the original one:

 - change int to unsigned when dealing with preset/band indices;
 - initialise the preamp and bands callbacks in the equalizer module, not the audio output, first checking for the existence of the variables;
 - implement a simple reference count for the equalizer (it seemed like a reasonable thing to do);
 - use the media player lock when getting/setting the equalizer reference on the media player instance.

I used the getting/setting of a libvlc_media_t instance on a media player as the pattern when implementing the reference count and the lock.

I will address the comments regarding my other patch containing the test case later, if this patch or a follow-up is finally approved.