Re-fix some warnings about mixing signed and unsigned better...

... Assuming that large unsigned magnitudes with high order bit set are not
the problem, but signed negatives of small magnitude may be:

1) Always cast the unsigned to signed in comparisons, not the other way.

Also:

2) Cast unsigned TERM to signed by itself, before subtracting.  Don't cast
the result.

3) Rewrite some comparisons by moving subtracted term to other side.

See commits
d2fe7b1757
f463eda36c

src/export/ExportMP3.cpp

@@ -1869,7 +1869,7 @@ ProgressResult ExportMP3::Export(AudacityProject *project,
 
          short *mixed = (short *)mixer->GetBuffer();
 
-         if (blockLen < (unsigned int)inSamples) {
+         if ((int)blockLen < inSamples) {
             if (channels > 1) {
                bytes = exporter.EncodeRemainder(mixed, blockLen, buffer.get());
             }