* Added 5th parameter to texvc call (can be null and defaults to rgb '1.0 1.0 1.0').

* Added 5th parameter to texvc call (can be null and defaults to rgb '1.0 1.0 1.0').
  This parameter controls texvc background color. Can be set to output transparent background
* Added $wgTexvcBackgroundColor parameter to DefaultSettings with value 'rgb 1.0 1.0 1.0'
* Changed Math.php to call texvc with new parameter