Fix jumping slider knob when it's size is fixed.

Source/Urho3D/UI/Slider.cpp

@@ -236,7 +236,7 @@ void Slider::UpdateSlider()
         {
             int sliderLength = (int)Max((float)GetWidth() / (range_ + 1.0f), (float)(border.left_ + border.right_));
 
-            if (knob_->IsFixedSize())
+            if (knob_->IsFixedWidth())
                 sliderLength = knob_->GetWidth();
 
             float sliderPos = (float)(GetWidth() - sliderLength) * value_ / range_;
@@ -253,7 +253,7 @@ void Slider::UpdateSlider()
         {
             int sliderLength = (int)Max((float)GetHeight() / (range_ + 1.0f), (float)(border.top_ + border.bottom_));
 
-            if (knob_->IsFixedSize())
+            if (knob_->IsFixedHeight())
                 sliderLength = knob_->GetHeight();
 
             float sliderPos = (float)(GetHeight() - sliderLength) * value_ / range_;