lavc: schedule AVCodecContext.lowres for removal on next bump.

It has been deprecated some time ago, but was forgotten during the last
bump.
lavf-regression: use -frames instead of -t for image tests

-t 0.5 is 12.5 frames at 25 fps, which may round to either 12 or 13 on
different platforms.
x86: float dsp: butterflies_float SSE

97c -> 49c
Some codecs could benefit from more unrolling, but AAC doesn't.