Support large files over 2GB on Windows (#1089)

Backport of 4dcf0b41c6
b01f45e141
https://github.com/taglib/taglib/pull/77

Tested with files larger than 2GB which have been created using

sox -n -r 44100 -C 320 large.mp3 synth 58916 sine 440 channels 2
sox -n -r 44100 -C 0 large.flac synth 25459 sine 440 channels 2
sox -n -r 44100 -C 10 large.ogg synth 229806 sine 440 channels 2
sox -n -r 44100 large.wav synth 6692 sine 440 channels 2
ffmpeg -f lavfi -i "sine=frequency=440:duration=244676" -y large.m4a

The only file which was readable with the tagreader example
before this commit was large.ogg. The problem is that long on
Windows is only 32-bit (also in LLP64 data model of 64-bit
compilation target) and all the file offsets using long are
too small for large files. Now long is replaced by offset_t
(defined to be long long on Windows and off_t on UNIX) for such
cases and some unsigned long are now size_t, which has the
correct size even on Windows.

taglib/xm/xmfile.cpp

@@ -591,7 +591,7 @@ void XM::File::read(bool)
     unsigned int count = 4 + instrument.read(*this, instrumentHeaderSize - 4U);
     READ_ASSERT(count == std::min(instrumentHeaderSize, (unsigned long)instrument.size() + 4));
 
-    long offset = 0;
+    offset_t offset = 0;
     if(sampleCount > 0) {
       unsigned long sampleHeaderSize = 0;
       sumSampleCount += sampleCount;