ext/ffmpeg/gstffmpegcodecmap.c: add flv encoder, fix #309050

ext/ffmpeg/gstffmpegcodecmap.c: add flv encoder, fix #309050

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcodecmap.c: (gst_ffmpeg_formatid_to_caps),
(gst_ffmpeg_formatid_get_codecids), (gst_ffmpeg_caps_to_codecid):
add flv encoder, fix #309050

* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_frame):
set both DELTA_UNIT and KEY_UNIT

* ext/ffmpeg/gstffmpegmux.c: (gst_ffmpegmux_loop):
use DELTA_UNIT and not the deprecated KEY_UNIT
fix #309049