Fix rounding on the pixel grid for negative values

Only affected border left and top pixels, with the thinkness of the border depending on the width of the kernel and the scaling factor.

Actual visible difference is probably next to non-existent.

Author: Frechdachs

src/descale.c

@@ -269,18 +269,16 @@ static double calculate_weight(enum DescaleMode mode, int support, double distan
 }
 
 
-// Taken from zimg https://github.com/sekrit-twc/zimg
+// Taken from zimg
+// https://github.com/sekrit-twc/zimg/blob/09802b8751c18165519d32407c498f0e3024f1f1/src/zimg/resize/filter.cpp#L33
 static double round_halfup(double x)
 {
     /* When rounding on the pixel grid, the invariant
      *   round(x - 1) == round(x) - 1
      * must be preserved. This precludes the use of modes such as
      * half-to-even and half-away-from-zero.
      */
-    bool sign = (x < 0);
-
-    x = round(fabs(x));
-    return sign ? -x : x;
+    return x < 0 ? floor(x + 0.5) : floor(x + 0.49999999999999994);
 }