Draft: libvlc: avoid using POSIX ssize_t in libvlc

ssize_t is not a standard C type. In some cases it's supposed to hold no more than a long. That's 2 GB in normal case which is enough for 16k*16k 16-bit RGBA.

The implementation shall support one or more programming environments in which the widths of blksize_t, pid_t, size_t, ssize_t, and suseconds_t are no greater than the width of type long. The names of these programming environments can be obtained using the confstr() function or the getconf utility.

We could use an int64_t to allow bigger sizes but allowing more than 2 GB of memory data on 32 bits system is not going to work well.