libavformat/img2.h: New field export_path_metadata to
VideoDemuxData to only allow the use of the extra metadata
upon explicit user request, for security reasons.

libavformat/img2dec.c: Modify image2 demuxer to make available
two special metadata entries called lavf.image2dec.source_path
and lavf.image2dec.source_basename, which represents, respectively,
the complete path to the source image for the current frame and
the basename i.e. the file name related to the current frame.
These can then be used by filters like drawtext and others. The
metadata fields will only be available when explicitly enabled
with image2 option -export_path_metadata 1.

doc/demuxers.texi: Documented the new metadata fields available
for image2 and how to use them.

doc/filters.texi: Added an example on how to use the new metadata
fields with drawtext filter, in order to plot the input file path
to each output frame.

Usage example:

ffmpeg -f image2 -export_path_metadata 1 -pattern_type glob
 -framerate 18 -i '/path/to/input/files/*.jpg'
 -filter_complex drawtext="fontsize=40:fontcolor=white:
 fontfile=FreeSans.ttf:borderw=2:bordercolor=black:
 text='%{metadata\:lavf.image2dec.source_basename\:NA}':x=5:y=50"
 output.avi

Fixes #2874.
Signed-off-by: Alexandre Heitor Schmidt <alexandre.schmidt@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>