fate/ffmpeg: add a -threads input option to the loopback decoder

Honor the requested value passed when calling make fate.

Signed-off-by: James Almer <jamrial@gmail.com>

tests/fate/ffmpeg.mak

@@ -257,7 +257,7 @@ FATE_FFMPEG-$(call REMUX, RAWVIDEO) += fate-ffmpeg-streamcopy-t
 
 # Test loopback decoding and passing the output to a complex graph.
 fate-ffmpeg-loopback-decoding: tests/data/vsynth_lena.yuv
-fate-ffmpeg-loopback-decoding:  CMD = transcode \
+fate-ffmpeg-loopback-decoding: CMD = transcode \
     "rawvideo -s 352x288 -pix_fmt yuv420p" $(TARGET_PATH)/tests/data/vsynth_lena.yuv nut \
-    "-map 0:v:0 -c:v mpeg2video -f null - -flags +bitexact -dec 0:0 -filter_complex '[0:v][dec:0]hstack[stack]' -map '[stack]' -c:v ffv1" ""
+    "-map 0:v:0 -c:v mpeg2video -f null - -flags +bitexact -threads $(THREADS) -dec 0:0 -filter_complex '[0:v][dec:0]hstack[stack]' -map '[stack]' -c:v ffv1" ""
 FATE_FFMPEG-$(call ENCDEC2, MPEG2VIDEO, FFV1, NUT, HSTACK_FILTER PIPE_PROTOCOL FRAMECRC_MUXER) += fate-ffmpeg-loopback-decoding