cap_ffmpeg: drop support for very old libav* versions

All relevant distros now ship the equivalent of Libav 0.8 or newer, so
drop support for everything older as it is a maintenance burden.
cap_ffmpeg: drop the local copy of the RIFF FourCC list

It is now exported by libavformat through a public API, so there is no
reason to carry an outdated custom copy.
cap_ffmpeg: wrap a forgotten instance of CODEC_ID_H264 in CV_CODEC()
cap_ffmpeg: do not use AVStream.r_frame_rate

r_frame_rate is a guessed number defined as "the least common multiple
of all framerates in the stream". It has been deprecated and removed in
new Libav releases.

Use avg_frame_rate instead, which is the average framerate of the
stream.
cap_ffmpeg: use avcodec_encode_video2() where available

avcodec_encode_video() has been deprecated and removed in newer
libavcodec versions.