Treat fonts with the same font descriptor, encoding and unicode map as aliases

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
and the same toUnicode map. 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.