filter_kernels: fix nearest scaler

The previous commit assumed the filter would be 1x1 (then constant
weight is correct) - but our code in fact uses at least a 2x2 filter. A
1x1 filter would generally be useless, except for nearest scaling - so
it didn't exist.

Insteasd of adding such a 1x1 filter, just turn the nearest weight
function into a scare function, which should take care of the issue.

video/out/filter_kernels.c

@@ -108,7 +108,7 @@ typedef struct filter_kernel kernel;
 
 static double nearest(kernel *k, double x)
 {
-    return 1.0;
+    return x > 0.5 ? 0.0 : 1.0;
 }
 
 static double bilinear(kernel *k, double x)