MythMusic: fix the goom visualizer

* use the correct image format to convert the Goom buffer to a QImage
* re-enable the scale factors so you can reduce the cpu utililization
  Increasing the scale factors will reduce the amount of data to be processed
  by Goom but that will also make the image more blocky.
* increase the maximum scale factor to 4 (MythGoom allows 1, 2 or 4 to be used)

Fixes #11023.