LibGUI: Fix slightly wrong value slider knob position

When clicking the value slider bar, the knob was not positioned as
close to the mouse as expected. When moving the mouse right, the knob
would lag a bit behind, and the cursor would eventually be outside the
knob.

The bug was due to knob_rect() taking knob_thickness into account to
prevent the knob from protruding outside the bar. The value_at(pos)
calculated the relative position based on the entire width of the bar.
This discrepancy is fixed by taking knob_thickness into account in
value_at(position).

Userland/Libraries/LibGUI/ValueSlider.cpp

@@ -139,11 +139,14 @@ Gfx::IntRect ValueSlider::knob_rect() const
 
 int ValueSlider::value_at(Gfx::IntPoint position) const
 {
-    if (position.x() < bar_rect().left())
+    int knob_thickness = knob_length();
+    float leftmost_knob_center = (float)bar_rect().left() + (float)knob_thickness / 2;
+    if (position.x() < leftmost_knob_center)
         return min();
-    if (position.x() > bar_rect().right())
+    float rightmost_knob_center = (float)bar_rect().right() - (float)knob_thickness / 2;
+    if (position.x() > rightmost_knob_center)
         return max();
-    float relative_offset = (float)(position.x() - bar_rect().left()) / (float)bar_rect().width();
+    float relative_offset = (float)(position.x() - leftmost_knob_center) / (rightmost_knob_center - leftmost_knob_center);
 
     int range = max() - min();
     return min() + (int)roundf(relative_offset * (float)range);