Always require a mask to get a temporary filename

Always require a mask to get a temporary filename

If no mask is supplied or the mask is attached to the end of the filename, we end up with unexpected names like
  <system-temp-dir>\lyx_tmpdir.qHp780.vcr780_<mask>
instead of a temporary file in the lyx temporary directory like
  <system-temp-dir>\lyx_tmpdir.qHp780\<mask>.vcr780.

(cherry picked from commit 5e4e4b674c00bc546213d42258f19e13a2b28bc3)