Modified SetValue to do nothing when the value is already set.

This seems also be done it BeOS R5 and does remove the constant 
flickering in the MediaPlayer slider.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19868 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/kits/interface/Control.cpp

@@ -298,6 +298,9 @@ BControl::Label() const
 void
 BControl::SetValue(int32 value)
 {
+	if (value == fValue)
+		return;
+
 	fValue = value;
 	Invalidate();
 }