Change sampleCount arguments, variables, return values to size_t...

... whenever they really describe the size of a buffer that fits in memory, or
of a block file (which is never now more than a megabyte and so could be fit in
memory all at once), or a part thereof.

src/effects/Fade.cpp

@@ -75,7 +75,7 @@ bool EffectFade::ProcessInitialize(sampleCount WXUNUSED(totalLen), ChannelNames
    return true;
 }
 
-sampleCount EffectFade::ProcessBlock(float **inBlock, float **outBlock, sampleCount blockLen)
+size_t EffectFade::ProcessBlock(float **inBlock, float **outBlock, size_t blockLen)
 {
    float *ibuf = inBlock[0];
    float *obuf = outBlock[0];