minedmap: separate collection for region list from preprocessing

Preparation for parallel processing, as well as a fix for regions
missing from later steps when the initial processing failed (rather than
using the processed data from a previous run).

src/bin/minedmap/common.rs

@@ -85,6 +85,11 @@ impl Config {
 		}
 	}
 
+	pub fn region_path(&self, coords: TileCoords) -> PathBuf {
+		let filename = coord_filename(coords, "mca");
+		[&self.region_dir, Path::new(&filename)].iter().collect()
+	}
+
 	pub fn processed_path(&self, coords: TileCoords) -> PathBuf {
 		let filename = coord_filename(coords, "bin");
 		[&self.processed_dir, Path::new(&filename)].iter().collect()