Treat fonts with the same font descriptor and encoding as aliases

Fix for issue #4339

Different fonts can point to the same font descriptor
(see https://github.com/mozilla/pdf.js/issues/4339 for details). With this
commit such fonts are treated as aliases if they have also the same encoding.
The according info is stored on the font descriptor. This change must also
ensure that aliases use always the same font name because translated fonts
can get cleared depending on the CLEANUP_TIMEOUT setting.