[client] wayland: use round half away from zero behaviour

In fractional-scale-v1, the scaling is defined as follows:

> For toplevel surfaces, the size is rounded halfway away from zero.

Previously, it is undefined. This commit makes waylandScaleMulInt follow
the round half away from zero behaviour.

client/displayservers/Wayland/scale.h

@@ -71,7 +71,7 @@ static inline int waylandScaleCeil(struct WaylandScale scale)
 
 static inline int waylandScaleMulInt(struct WaylandScale scale, int value)
 {
-  return (int)(((int64_t)value * scale.num) / scale.den);
+  return (int)(((int64_t)value * scale.num + scale.den / 2) / scale.den);
 }
 
 static inline double waylandScaleToDouble(struct WaylandScale scale)