Fix bug with 'space' key on brush tool

Since recent change in the way we handle the 3d gestures, we have to
make sure we update the snap offset from within the gesture handler.

src/tools/brush.c

@@ -179,6 +179,10 @@ static int on_hover(gesture3d_t *gest)
         return 0;
     }
 
+    // Update snap offset in case we press 'space'.
+    gest->snap_offset = goxel.snap_offset * goxel.tool_radius +
+                        ((painter->mode == MODE_OVER) ? 0.5 : -0.5);
+
     if (shift)
         render_line(&goxel.rend, brush->start_pos, gest->pos, NULL, 0);