Added an attribute string_id_to_buffer, which gives the offset you

Added an attribute string_id_to_buffer, which gives the offset you
have to add to the id() of a string to get the address of the
data. This is a horrible and error-prone hack, but unfortunately it is
one that is needed for playing audio and crearing pixmaps. Having the
constant here is at least marginally better than the previous practice
of having the constant 16 all over the place...