x86: Initialize mmxext after amd3dnow optimizations

The mmxext optimizations should be at least equally fast if available and
amd3dnow optimizations are being deprecated. Thus the former should
override the latter, not the other way around.
mpeg4videodec: move use_intra_dc_vlc from MpegEncContext to Mpeg4DecContext
mpegvideo: remove an unused variable
mpeg12dec: move first_slice from MpegEncContext to Mpeg1Context
mpeg4videodec: move intra_dc_threshold from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move showed_packed_warning from MpegEncContext to Mpeg4DecContext
mpegvideo: don't copy input_picture_number in update_thread_context()

It is encoding-only, frame threading is not used for encoding.
mpegvideo: move setting encoding-only vars from common_defaults() to encode_defaults()
mpegvideo: move encoding-only initialization from common_init() to encode_init()
mpegvideo: do not update last_non_b_pict_type in update_thread_context()

It is used for encoding only.
mpegvideo: split the encoding-only parts of ff_MPV_frame_end() into a separate function
mpegvideo: remove commented out cruft from ff_MPV_frame_end()
mpegvideo: move encode-only parts of common_end() to encode_end()
mpegvideo: make frame_size_alloc() static.

It is not called from outside of mpegvideo.c anymore.
mpegvideo: make ff_release_unused_pictures() static

It is only called from one place in mpegvideo.c now. Also remove the
remove_current parameter, which is always 1.
vc1dec: move setting repeat_pict after frame_start() has been called.

This will allow removing the hacks where each decoder sets
current_picture_ptr on its own.
FATE: drop the last partial frame in the wmv8-drm test

This has already been done in eb066a4ce9e82cddb8f5af574804aa9889450b34,
but the number of frames produced from that file changed due to
timestamps changes since then.
mpegvideo: do not set current_picture_ptr in decoders

This code was originally added in
5f1948111a4920ff74e31b4284ab0200f7938539 to h263 to set decoded frame
pts to some random numbers (removed in
a1c5cc429d99216406170eac7e8352860076d3e8) and then cargo culted to other
decoders.

The code is left in h263dec for now, since some part of the decoder
(apparently OBMC) relies on the specific previous frame to be reused.
mjpegenc: write the JFIF header if the sample aspect ratio is set

MpegEncContext.aspect_ratio_info is never set for mjpeg, so this was
never written before.
mjpegenc: cosmetics, reformat jpeg_put_comments()