Allow rendering the graph on a background thread (#1014)

* Render on background thread

* More work

* variable fix

* Add comments

src/Calculator/Views/GraphingCalculator/EquationInputArea.xaml.cpp

@@ -330,9 +330,6 @@ void EquationInputArea::SubmitTextbox(TextBox ^ sender)
     {
         val = validateDouble(sender->Text, variableViewModel->Value);
         variableViewModel->Value = val;
-
-        // Assign back to val in case it gets changed due to min/max
-        val = variableViewModel->Value;
     }
     else if (sender->Name == "MinTextBox")
     {