corrected error moving temp image file

Avoided error "OSError: [Errno 18] Invalid cross-device link" by using
shell command instead (move for windows, mv for others). The error seems
to be related to moving files between different filesystems using
os.rename in Python, according to search results.

singleimage.py

@@ -115,7 +115,21 @@ class ChunkymapOfflineRenderer:
                 if os.path.isfile(dest_png_path):
                     os.remove(dest_png_path)
                 print("Moving temp image from "+tmp_png_path+" to "+dest_png_path+"...")
-                os.rename(tmp_png_path, dest_png_path)
+                
+                move_cmd_string = "mv"
+                if os_name=="windows":
+                    move_cmd_string= "move"
+                this_move_cmd_string = move_cmd_string+" \""+tmp_png_path+"\" to \""+dest_png_path+"\"..."
+                subprocess.call(this_move_cmd_string, shell=True)
+                # os.rename(tmp_png_path, dest_png_path)  # fails with the following output:
+                # Moving temp image from /home/owner/chunkymap/chunkymap-genresults/FCAGameAWorld/singleimage.png to /var/www/html/minetest/chunkymapdata/worlds/FCAGameAWorld/singleimage.png...
+                # Traceback (most recent call last):
+                #  File "chunkymap/singleimage.py", line 157, in <module>
+                #     cmor.RenderSingleImage()
+                #   File "chunkymap/singleimage.py", line 118, in RenderSingleImage
+                #     os.rename(tmp_png_path, dest_png_path)
+                # OSError: [Errno 18] Invalid cross-device link
+                
                 final_png_path = dest_png_path
             print("Png image saved to:")
             print("  "+final_png_path)