avfilter/vf_edgedetect: properly implement double_threshold()

Important part of this algorithm is the double threshold step: pixels
above "high" threshold being kept, pixels below "low" threshold dropped,
pixels in between (weak edges) are kept if they are neighboring "high"
pixels.

The weak edge check uses a neighboring context and should not be applied
on the plane's border. The condition was incorrect and has been fixed in
the commit.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>
Reviewed-by: Andriy Gelman <andriy.gelman@gmail.com>

libavfilter/vf_edgedetect.c

@@ -294,7 +294,7 @@ static void double_threshold(int low, int high, int w, int h,
                 continue;
             }
 
-            if ((!i || i == w - 1 || !j || j == h - 1) &&
+            if (!(!i || i == w - 1 || !j || j == h - 1) &&
                 src[i] > low &&
                 (src[-src_linesize + i-1] > high ||
                  src[-src_linesize + i  ] > high ||

tests/ref/fate/filter-edgedetect

@@ -1 +1 @@
-edgedetect          93ceace33f6636bcdbeb037317c65745
+edgedetect          04ff46bb35edff3dbad4102391516d25

tests/ref/fate/filter-edgedetect-colormix

@@ -1 +1 @@
-edgedetect-colormix 1b8658252e2f03fbae30e6d63dd24c7c
+edgedetect-colormix 9f50c5586f899a8f5a10059154d64bde