concatdec: allow to set explicitly file input format
concatdec: add force_duration option

When set to 1, demuxer stops reading file when timestamp reaches stated duration
concatdec: update doc
Add av_bprint_fd_contents()
lavfi indev: allow non-mmappable files for graph_file

Use av_bprint_fd_contents() instead of av_file_map()
drawtext: add fontcolor_expr

An option for fontcolor, dynamically evaluated