scheme: better colours + parallel generation

Fix some wallpapers not having enough colours by disabling filter (also gives better colours)
Use GNU parallel to run scheme generation in parallel
Also ensure all schemes exist when generating
Cache wallpaper thumbnails

scheme/score.py

@@ -22,7 +22,7 @@ class Score:
     @staticmethod
     def score(colors_to_population: dict) -> list[int]:
         desired = 14
-        filter_enabled = True
+        filter_enabled = False
         dislike_filter = True
 
         colors_hct = []