app: fix offset handling in spiral gradients

In gimp:gradient, properly scale the "offset" property in spiral
graidnets.

(cherry picked from commit 04280690b2)

app/operations/gimpoperationgradient.c

@@ -777,6 +777,8 @@ gradient_calc_spiral_factor (gdouble   dist,
       gdouble ang;
       double  r;
 
+      offset = offset / 100.0;
+
       ang0 = atan2 (axis[0], axis[1]) + G_PI;
       ang1 = atan2 (x, y) + G_PI;