Yep, I looked at exactly that but can't make anything out of it. I don't know anything about the Mackie controller, so it's probably something quite spesific to that...
The m_mackie_modifiers variable seems to be manipulated in these parts of the code :
Code:
if (evt->midi_message[1] >= 0x46 && evt->midi_message[1] < 0x4A)
{
int mask=(1<<(evt->midi_message[1]-0x46));
if (evt->midi_message[2] >= 0x40)
m_mackie_modifiers|=mask;
else
m_mackie_modifiers&=~mask;
}