Bug 760737 - Brush angle jumps when tablet pen is tilted horizontally.

Fix cursor rotation jump when tablet pen is tilted horizontally.
This changes to the correct values for the special case of tilt_x == 0.0.

Reviewer (Jehan)'s note: computing the convergence of the tilt function
in the `else` block, when tilt_x approaches 0, tilt value indeed
converges to 0.25 with tilt_y > 0 and 0.75 (or -0.25 which is the same)
with tilt_y < 0.

app/core/gimpdynamicsoutput.c

@@ -570,9 +570,9 @@ gimp_dynamics_output_get_angular_value (GimpDynamicsOutput *output,
       if (tilt_x == 0.0)
         {
           if (tilt_y >= 0.0)
-            tilt = 0.5;
+            tilt = 0.25;
           else if (tilt_y < 0.0)
-            tilt = 0.0;
+            tilt = 0.75;
           else
             tilt = -1.0;
         }