NAME
       LoadXtra -- load a source file's .xtra file into memory

   SYNOPSIS
       xtraFile = LoadXtra(fileName);
       D0                  A0

       struct BlitzXtra * xtraFile = LoadXtra(char * fileName);

   FUNCTION
       This function will allocate memory for and load into this memory a Blitz
       .xtra file.

   INPUTS
       fileName - a pointer to a null-terminated string holding the name of the
                  .xtra file to load

   RESULT
       xtraFile - a pointer to the associated BlitzXtra structure (or NULL for
                  failure)

   NOTES
       No checking is made to ensure that the file passed is a valid .xtra file