Add pixel size test

Author: MyreMylar

src_c/transform.c

@@ -4240,6 +4240,12 @@ pixelate(pgSurfaceObject *srcobj, pgSurfaceObject *dstobj, int pixel_size)
         newsurf = pgSurface_AsSurface(dstobj);
     }
 
+    if (pixel_size < 1) {
+        return (SDL_Surface *)(RAISE(
+            PyExc_ValueError,
+            "pixel_size must be greater than 0."));
+    }
+    
     if (newsurf->w != src->w || newsurf->h != src->h) {
         return (SDL_Surface *)(RAISE(
             PyExc_ValueError,