make PADOFFSET be SSizet_t

make PADOFFSET be SSizet_t

Currently it's defined as U32 or U64 depending on whether pointers are
32 bit or 64-bit, which is just a long-winded way of doing

    typedef Size_t PADOFFSET

Change it to

    typedef SSize_t PADOFFSET

Making it signed makes it easier to handle comparisons against PADOFFSET
values that can be -1, such as PL_comppad_name_floor (which will be fixed
in the next commit).