update getLowerBound and getUpperBound to check startPoint prop (#683)

* update getLowerBound and getUpperBound to check startPoint prop

* more understandable code

Co-authored-by: michael kantz <michael@3dcastor.com>

src/Slider.tsx

@@ -183,10 +183,14 @@ class Slider extends React.Component<SliderProps, SliderState> {
   }
 
   getLowerBound() {
-    return this.props.min;
+    const minPoint = this.props.startPoint || this.props.min;
+    return this.state.value > minPoint ? minPoint : this.state.value;
   }
 
   getUpperBound() {
+    if (this.state.value < this.props.startPoint) {
+      return this.props.startPoint;
+    }
     return this.state.value;
   }