Because only the unused channelVolueAdjust value was clipped, the actual
velocity of the note would often overflow, causing the note to be almost
muted instead of played at full volume.

I assume it was an oversight that chanelVolumeAdjust wasn't used, so
I've fixed that and moved the clipping to after all adjustments have
been made. Also eliminated some unneeded floating-point math.

Is it correct now? I honestly don't know. It seems that, at least in
Waxworks, almost all notes end up being played at max volume. But maybe
that's how it should be?

After feedback from NMIError, and some more guesswork. The master volume
isn't applied until noteOnSetVolume(), so that the correct adjusted
volume can be stored in the channel data structure.

Now the volume seems too low to me, but of course that could be fixed by
having some other default value than 15 for _masterVolume.

At this point, the driver still tries to play notes at the maximum
allowed volume or louder, but ScummVM's music volume setting can bring
them back down to the expected range. That may have to be good enough
for now.

Also, since the master volume is only used internally there's no need to
keep converting back and forth. Just use the interval 0-255 throughout.