create JPG and 8bit PNG under temporary name first, rename when done

this way files only show up for download in the web interface when
really fully done, and not while still being written to

scripts/render.py

@@ -404,7 +404,8 @@ class JobRenderer(threading.Thread):
                 img = Image.open(prefix + '.png')
                 try:
                     img = img.convert('RGB')
-                    img.save(prefix + '.jpg', quality=50)
+                    img.save(prefix + '.jpg.tmp', quality=50)
+                    os.rename(prefix + '.jpg.tmp', prefix + '.jpg')
                 except Exception as e:
                     LOG.warning("PNG to JPEG conversion failed: %s" % e)
                 img.thumbnail((200, 200), Image.ANTIALIAS)
@@ -414,9 +415,10 @@ class JobRenderer(threading.Thread):
             img.close()
 
             try:
-                pngquant_cmd = [ "pngquant", "--output", "%s.8bit.png" % prefix,
+                pngquant_cmd = [ "pngquant", "--output", "%s.8bit.tmp" % prefix,
                                  "%s.png" % prefix ]
                 subprocess.check_call(pngquant_cmd)
+                os.rename(prefix + '.8bit.tmp', prefix + '8bit.png')
             except Exception as e:
                 LOG.warning("PNG color reduction failed: %s" % e)