audio/format: fix doublep sample format

This was accidentally equivalent to floatp.
audio/format: add heuristic to estimate loss on format conversion

The added function af_format_conversion_score() can be used to select
the best sample format to convert to in order to reduce loss and extra
conversion work.

It calculates a "loss" score when going from one format to another, and
for each conversion that needs to be done a certain score is subtracted.
Thus, if you have to convert from one format to a set of other formats,
you can calculate the score for each conversion, and pick the one with
the highest score.

Conversion between int and float is considered the worst case. One odd
consequence is that when converting from s32 to u8 or float, u8 will be
picked.

Test program used to develop this follows:

#define MAX_FMT 200
struct entry {
    const char *name;
    int score;
};

static int compentry(const void *px1, const void *px2)
{
    const struct entry *x1 = px1;
    const struct entry *x2 = px2;
    if (x1->score > x2->score)
        return 1;
    if (x1->score < x2->score)
        return -1;
    return 0;
}

int main(int argc, char *argv[])
{
    for (int n = 0; af_fmtstr_table[n].name; n++) {
        struct entry entry[MAX_FMT];
        int entries = 0;
        for (int i = 0; af_fmtstr_table[i].name; i++) {
            assert(i < MAX_FMT);
            entry[entries].name = af_fmtstr_table[i].name;
            entry[entries].score =
                af_format_conversion_score(af_fmtstr_table[i].format,
                                           af_fmtstr_table[n].format);
            entries++;
        }
        qsort(&entry[0], entries, sizeof(entry[0]), compentry);
        for (int i = 0; i < entries; i++) {
            printf("%s -> %s: %d \n", af_fmtstr_table[n].name,
                   entry[i].name, entry[i].score);
        }
    }
}
ao_lavc: use af_format_conversion_score()

This should allow it to select better fallback formats, instead of
picking the first encoder sample format if ao->format is not equal to
any of the encoder sample formats.

Not sure what is supposed to happen if the encoder provides no
compatible sample format (or no sample format list at all), but in this
case ao_lavc.c still fails gracefully.
encode_lavc: add missing newline in error message
demux: reset EOF flag differently

This should be almost equivalent, but is slightly better because the
EOF flag is reset earlier.
demux_lavf: remove broken and commented byte based seeks

This was originally added for better seeking where libavformat's seek
function won't work well: files with timestamp resets. In these cases,
the code tried to calculate an average bitrate, and then do byte based
seeks by multiplying the seek target time with the bitrate.

Apparently this was unreliable enough that the code was just commented
(and other parts became inactive). Get rid of it.

Note that the player still does byte based seeks in these cases when
doing percent-seeks.
demux: simplify handling of filepos field

demuxer->filepos contains the byte offset of the last read packet. This
is so that the player can estimate the current playback position, if no
proper timestamps are available. Simplify it to use demux_packet->pos in
the generic demuxer code, instead of bothering every demuxer
implementation about it.

(Note that this is still a bit incorrect: it relfects the position of
the last packet read by the demuxer, not that returned to the user. But
that was already broken, and is not that trivial to fix.)
demux: remove unused commands

These were replaced with DEMUXER_CTRL_SWITCHED_TRACKS a while ago.
demux: update a comment
demux_packet: add source stream index

Might be helpful later.