LibPDF: Implement support for callgsubr in CFF font programs

Font programs are bytecode programs defining glyphs. If several glyphs
share a piece of outline, that opcode sequence can be put in a
subroutine ("subr") table and the definition of those glyphs can then
call that subroutine by number, to reduce file size.

CFF fonts can in theory contain multiple fonts, and so there's a global
subr table shared by all the fonts in one CFF, and a local per-fornt
subr table.  We used to only implement the local subr table, now we
implement both.

(We only support one font per CFF, and at least in PDF files, that's
all that's ever used. So a global subr table isn't very useful.
But the spec explicitly allows it -- "Global subroutines may be used in
a FontSet even if it only contains one font." -- and it happens in
practice.)

Userland/Libraries/LibPDF/Fonts/PS1FontProgram.cpp

@@ -92,7 +92,7 @@ PDFErrorOr<void> PS1FontProgram::parse_encrypted_portion(ByteBuffer const& buffe
                 reader.move_by(encrypted_size);
                 auto glyph_name = word.substring_view(1);
                 GlyphParserState state;
-                TRY(add_glyph(glyph_name, TRY(parse_glyph(line, subroutines, state, false))));
+                TRY(add_glyph(glyph_name, TRY(parse_glyph(line, subroutines, {}, state, false))));
             }
         }
     }