Replaced two echo sequences in the header with heredocs
- replaced all backtick subshell invocations (`...`) with the
  recommended parenthetical syntax ($( ... ))

- split a couple of longer subshells across multiple lines to try
  to shorten long horizontal line spans

- doublequoted every variable that was 1- in a position to undergo
  expansion, 2- could potentially, at configuration time or at
  runtime, contain whitespace, and 3- did not look like whitespace
  splitting could be significant to the program execution
Edited the getopts block and variable initialization for terseness
- added debug_vars(), msg(), and exit_msg() to make it easier to
  manage output being written to stderr during program execution
  and just prior to exiting under error conditions

- replaced calls of "echo ... >&2" at every point in the codebase
  with appropriate calls to debug_vars(), msg(), and exit_msg()
- replaced deprecated single-bracket "if [ ... ] ; ..." conditional
  syntax with more modern "if [[ ... ]] ; ..." syntax, and removed
  the doublequotes from variables which no longer need protection
  from variable expansion

- replaced string comparison conditionals with the arithmetic form
  ("if ((...)) ; ...") wherever possible
- replaced literal directory paths with paths relative to $HOME

- cleaned up $TmpDir initialization logic and messaging
- replaced unnecessary conditionals evaluating $? with conditionals
  hooked directly on the execution of the command in question where
  applicable

- tightened up vertical whitespace in the areas surrounding calls
  to the MKV tools
Replaced getFullPathDir() and the variables that supported it with
a quick bit of jiggery-pokery on $inputFile and $outputFile to
dereference potential relative paths into absolute paths.

The jiggery-pokery in question involves a single subshell and some
string manipulation instead of the more intuitive solution of just
calling out to "readlink -ne", because that is apparently common
only on GNU systems; the solution with "cd" and string manipulation
should require only bash, keeping the dependency footprint low.