Actually use the argument in convert

conquest/regioner.py

@@ -549,10 +549,16 @@ class MapMaker(ConquestMap):
         return points
 
     async def convert_masks(self, regions):
-        async for mask_path in AsyncIter(self.masks_path().iterdir()):
-            # Don't both checking if masks are in self.regions
-            img: Image.Image = Image.open(mask_path).convert(MASK_MODE)
-            img.save(mask_path, "PNG")
+        if regions:
+            async for region in regions:
+                mask_path = self.masks_path() / f"{region}.png"
+                img: Image.Image = Image.open(mask_path).convert(MASK_MODE)
+                img.save(mask_path)
+        else:
+            async for mask_path in AsyncIter(self.masks_path().iterdir()):
+                # Don't both checking if masks are in self.regions
+                img: Image.Image = Image.open(mask_path).convert(MASK_MODE)
+                img.save(mask_path, "PNG")
         return True
 
     async def prune_masks(self):