tests/fate/filter-video: add tests for vignette backward mode

Use angle 0, so the gain is exactly 1 inside dmax, and move the center
to the top left corner so that part of the frame lies beyond dmax and
must saturate. Cover the planar (yuv444p) and packed (rgb24) paths.

Signed-off-by: Martin Mirchev <mmirchev@comp.nus.edu.sg>

tests/fate/filter-video.mak

@@ -480,6 +480,9 @@ fate-filter-chromashift-wrap:  CMD = framecrc -lavfi testsrc2=r=5:d=1,chromashif
 FATE_FILTER-$(call FILTERFRAMECRC, TESTSRC2 FPS DECIMATE) += fate-filter-decimate
 fate-filter-decimate: CMD = framecrc -lavfi testsrc2=r=24:d=10,fps=60,decimate=5,decimate=4,decimate=3 -pix_fmt yuv420p
 
+FATE_FILTER-$(call FILTERFRAMECRC, TESTSRC2 FORMAT VIGNETTE) += fate-filter-vignette-backward-yuv444p fate-filter-vignette-backward-rgb24
+fate-filter-vignette-backward-%: CMD = framecrc -lavfi testsrc2=s=64x64:r=1:d=1,format=$(@:fate-filter-vignette-backward-%=%),vignette=a=0:x0=0:y0=0:mode=backward -pix_fmt $(@:fate-filter-vignette-backward-%=%)
+
 FATE_FILTER-$(call FILTERFRAMECRC, TESTSRC2 FPS MPDECIMATE) += fate-filter-mpdecimate
 fate-filter-mpdecimate: CMD = framecrc -lavfi testsrc2=r=2:d=10,fps=3,mpdecimate -pix_fmt yuv420p
 

tests/ref/fate/filter-vignette-backward-rgb24

@@ -0,0 +1,6 @@
+#tb 0: 1/1
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 64x64
+#sar 0: 1/1
+0,          0,          0,        1,    12288, 0x282440f2

tests/ref/fate/filter-vignette-backward-yuv444p

@@ -0,0 +1,6 @@
+#tb 0: 1/1
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 64x64
+#sar 0: 1/1
+0,          0,          0,        1,    12288, 0xf08ed17c