avcodec/movtextenc: Simplify writing to AVBPrint

avcodec/movtextenc: Simplify writing to AVBPrint

The mov_text encoder uses an AVBPrint to assemble the subtitles;
yet mov_text subtitles are not pure text; they also have a binary
portion that was mostly handled as follows:

    uint32_t size = /* calculation */;
    size = AV_RB32(&size);
    av_bprint_append_data(bprint, (const char*)&size, 4);

Here AV_RB32() is a no-op on big-endian systems and a LE-BE swap
on little-endian systems, making the output endian-independent.

Yet this is ugly and unclean: On LE systems, the variable size from
the snippet above won't contain the correct value any more. Furthermore,
using this pattern leads to lots of small writes to the AVBPrint.

This commit therefore changes this to using a temporary buffer instead:

    uint8_t buf[4];
    AV_WB32(buf, /* size calculation */);
    av_bprint_append_data(bprint, buf, 4);

This method also allows to use bigger buffers holding more than one
element, saving calls to av_bprint_append_data() and reducing codesize.

Reviewed-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>