atomic: prefer gcc builtins over win32 atomics, if available.

The mingw win32 atomics appear to be faulty, so they should not be used
if the gcc ones are available.

Signed-off-by: Martin Storsjö <martin@martin.st>
lavc: remove disabled FF_API_AVCODEC_RESAMPLE cruft
lavc: remove disabled FF_API_LIBMPEG2 cruft
lavc: remove disabled FF_API_MMI cruft
lavc: remove disabled FF_API_SNOW cruft
lavc: remove disabled FF_API_IDCT cruft
lavf: remove disabled FF_API_CLOSE_INPUT_FILE cruft
lavf: remove disabled FF_API_APPLEHTTP_PROTO cruft
lavf: remove disabled FF_API_READ_PACKET cruft
lavf: remove disabled FF_API_INTERLEAVE_PACKET cruft
lavf: remove disabled FF_API_AV_GETTIME cruft
lavf: remove disabled FF_API_R_FRAME_RATE cruft
mpegvideo: remove useless references to h264 and svq3

The h264 decoder does not use mpegvideo anymore. The svq3 decoder only
uses ff_draw_horiz_band().
mpegvideo: reindent
mpegvideo: remove FMT_H264
h264: remove a useless svq3 condition

The svq3 decoder does not call ff_h264_frame_start() anymore.
h264_refs: cosmetics, reformat
vf_gradfun: fix uninitialized variable use

CC:libav-stable@libav.org
vf_hqdn3d: fix uninitialized variable use

CC:libav-stable@libav.org
h264: check that DPB is allocated before accessing it in flush_dpb()