Revert "removing lowres support"

There have been multiple user complaints about loosing this feature
while its not clear the 3% speedloss claims where real or fabricated.
My own testing indicates no statistically significant speed difference
both with mpeg2 and mpeg4, and if at all the code with lowres support
is a tiny bit faster than without.

This reverts commit 92ef4be4, reversing
changes made to 2e07f429.

Conflicts:

	cmdutils.c
	libavcodec/arm/vp8dsp_init_arm.c
	libavcodec/mpegvideo.c
	libavcodec/mpegvideo.h
	libavutil/arm/Makefile

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>