mpegvideo_enc: add intra_penalty option for p frames

mpegvideo_enc: add intra_penalty option for p frames

This option allows more control over the use of intra macroblocks in
predictive frames.

By using '-intra_penalty max', intra macroblocks are never used in
predictive frames.

It is useful for glitch artists to generate input material. This option
allows them to split and merge two video files while maintaining fluid
motion from the second video without having intra macroblocks restoring
chunks of the first video.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>