DrawSlider() now locks the looper - this should fix bug #396.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@17002 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/kits/interface/Slider.cpp

@@ -716,6 +716,7 @@ BSlider::Draw(BRect updateRect)
 void
 BSlider::DrawSlider()
 {
+	if (LockLooper()) {
 #if USE_OFF_SCREEN_VIEW
 		if (!fOffScreenBits)
 			return;
@@ -734,6 +735,7 @@ BSlider::DrawSlider()
 			DrawBitmap(fOffScreenBits, B_ORIGIN);
 		}
 #endif
+	}
 }