avfilter/graphparser: allow specifying filter@id as filter instance

See http://lists.ffmpeg.org/pipermail/ffmpeg-user/2017-April/035975.html
Parsed_filter_X could remain and user can override it with custom one.

Example:
ffplay -f lavfi "nullsrc=s=640x360,
sendcmd='1 drawtext@top reinit text=Hello; 2 drawtext@bottom reinit text=World',
drawtext@top=x=16:y=16:fontsize=20:fontcolor=Red:text='',
drawtext@bottom=x=16:y=340:fontsize=16:fontcolor=Blue:text=''"

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>
