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/Repair.h

@@ -44,8 +44,9 @@ private:
 
    bool ProcessOne(int count, WaveTrack * track,
                    sampleCount start,
-                   sampleCount len,
-                   sampleCount repairStart, sampleCount repairLen);
+                   size_t len,
+                   size_t repairStart, // offset relative to start
+                   size_t repairLen);
 };
 
 #endif // __AUDACITY_EFFECT_REPAIT__