Fix timing of AVI files dumped on Linux

The timing information is set on s_scaled_frame->pts, giving precise
timing information to the encoder. Frames arriving too early (less than
one tick after the previous frame) are droped. The setting of packet's
timestamps and flags is done after the call to avcodec_encode_video2()
as this function resets these fields according to its documentation.
Merge pull request #1345 from sgadrat/fix-avidump-framerate

Fix timing of AVI files dumped on Linux