Reference used to be clever and stored the index of a ref in 18 bits
and the generation in 14 bits, so that both fit into a single u32.
However:
- It set MAX_REF_INDEX incorrectly (the max value of an 18-bit number
is `(1 << 18) - 1`, not `(1 << 19) - 1`
- pdf_reference_1-7.pdf has 349223 objects, and that's larger
than `(1 << 18) - 1` (which is 262143)
Since a Reference is stored in Value which is a Variant that also
stores a pointer, the size of Value is already 64-bit. So just don't
be clever here.
Makes pdf_reference_1-7.pdf get a bit further during decryption.
