[AD] Memfile support

[Lennart Steinke <lst@xxxxxxxxxx>]

Hello,

I'm currently working on a patch that will allow the packfile functions 
to work on a normal memory chunk by calling:

PACKFILE* pack_open_mem(void *data, int size);

(the size param is of type int, since the corresponding packfile member 
is an int as well - not sure if we should change both to size_t)

The actual handling works fine, but in order to make it useful I'd have 
to change most loading functions slightly.

Right now, most loaders do something like this:

FOO *load_foo(const chat* filename) {
  f = pack_fopen(filename, F_READ);
  if (!f)
     return NULL;
 /* ... */
}

Which would be needed to be changed into:

FOO* load_foo(const char *filename) {
 f = pack_fopen(filename, F_READ);
  if (f)
       return pack_load_foo(f);
  return NULL;
}

And the pack_load_foo() functions would need to be exposed.

This allows you to do two things:
a) You can create BITMAPs, SAMPLEs, etc. from memory
b) You can load these types also from the middle of a larger, concated file.

PACKFILE *f = pack_fopen("foo.dat", "rb");
pack_fseek(f, 1024);
bmp = pack_load_bitmap(f, NULL);

I think I can finish the changes during the weekend. What do you guys think?

Regards,
Lenny