Fix render problem on Slider (#15039)
Fix render problem on Slider (#15039)

Fixes a small rendering problem when dragging the slider, as well as a couple of small bugs (de-duping of value changes, and actually using a parameter passed to _unlerp).
cbracken/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter