Enable multi-threading at least for all demanding G'MIC filters

[ChameleonScales]

Actually, memorizing the selection would already be of huge help in that regard, so I made a feature request:

Feature request : Memorize the selection's position

When using the G’MIC Qt plugin for either GIMP or Krita, if you either change your selection or jump to another document tab before the filter is done processing, the resulting image will be pasted in whatever new selection or other document you’re in. (Note: For GIMP it only happens when you change the selection, so you can at least jump to another tab without fear). In my situation, having the plugin memorize the location of the filtered image to paste would be very (or extremely) useful. Her…