[trak] Fix calc

We were getting the first track record always.  Ie. this line:

       if (trackTable[i].get_track_value () == 0.f)
       {
-       trackTableEntry = &trackTable[0];
+       trackTableEntry = &trackTable[i];
        break;
       }

The rest is cleanup.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1263 for the most part.