Add necessary initialisation in test/mandelbrot

Detected by valgrind :
==16102== Conditional jump or move depends on uninitialised value(s)
==16102==    at 0x449A10: Drawing_Area::idle() (mandelbrot.cxx:143)

1 files changed, 1 insertions, 0 deletions

diff --git a/test/mandelbrot.h b/test/mandelbrot.h
index b946232f5..ccae9db11 100644
--- a/test/mandelbrot.h
+++ b/test/mandelbrot.h
@@ -63,6 +63,7 @@ public:
     scale = 4.0;
     iterations = 1<<DEFAULT_ITERATIONS;
     brightness = use_colors ? DEFAULT_BRIGHTNESS_COLOR : DEFAULT_BRIGHTNESS;
+    sx = sy = sw = sh = 0;
   }
   int idle();
 };