wrlib: change default scaling interpolation to Catmull-Rom

This patch is implementing a new default Catmull-Rom filter
to resize images.
Catmull-Rom is a special case of cubic interpolation with B=0 and
C=0.5 (in the Mitchell-Netravali formulation).
It provides slighlty sharper results than Mitchell with the same
performance.
Catmull-Rom is a better choice for a window manager as it prioritizes
sharpness, making small elements feel crisp.

wrlib/wraster.h.in

@@ -143,7 +143,8 @@ typedef enum {
 	RBellFilter,
 	RBSplineFilter,
 	RLanczos3Filter,
-	RMitchellFilter
+	RMitchellFilter,
+	RCatmullRomFilter
 } RScalingFilter;