filter_kernels: fix quadric window

3909e4cdfc seems to have replaced this 0.5 constant by 0.75, using its
presence in glumpy as justification.

0.75 is clearly a bug in glumpy, as its own source code doesn't even
match the comment immediately above it. Every other implementation of
this window I could find uses 0.5, including e.g. ImageMagick.

video/out/filter_kernels.c

@@ -199,7 +199,7 @@ static double hamming(params *p, double x)
 
 static double quadric(params *p, double x)
 {
-    if (x <  0.75) {
+    if (x <  0.5) {
         return 0.75 - x * x;
     } else if (x <  1.5) {
         double t = x - 1.5;