avformat/yuv4mpegenc: Simplify writing global and packet headers

avformat/yuv4mpegenc: Simplify writing global and packet headers

YUV4MPEG writes a string as header for both the file itself as well as
for every frame; these strings contain magic strings and these were up
until now included in the string to write via %s. Yet they are compile
time constants, so one can use the compile-time string concatentation
instead of inserting these strings at runtime.
Furthermore, the global header has been written via snprintf() to
a local buffer first before writing it. This can be simplified by using
avio_printf().

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>