Make many counts of tracks and channels unsigned...

... And in some places where a library uses signed types, assert that
the reported number is not negative.

What led me to this, is that there are many places where a size_t value for
an allocation is the product of a number of channels and some other number.

src/effects/Invert.cpp

@@ -54,12 +54,12 @@ bool EffectInvert::IsInteractive()
 
 // EffectClientInterface implementation
 
-int EffectInvert::GetAudioInCount()
+unsigned EffectInvert::GetAudioInCount()
 {
    return 1;
 }
 
-int EffectInvert::GetAudioOutCount()
+unsigned EffectInvert::GetAudioOutCount()
 {
    return 1;
 }