Fix maxWidth bug in Panel

Panel could pass a maxWidth to the child Render function that was greater than the maxWidth that was passed to the Panel render function.
I noticed this when trying to nest a table in a panel and the word wrapping was being disabled.

N.B It might be that Table.Measure ought to not return a measurment larger than maxWidth, but this Panel change seems defensivly sensible regardless.

src/Spectre.Console/Widgets/Panel.cs

@@ -99,6 +99,7 @@ namespace Spectre.Console
 
             var panelWidth = childWidth + edgeWidth;
             panelWidth = Math.Min(panelWidth, maxWidth);
+            childWidth = panelWidth - edgeWidth;
 
             var result = new List<Segment>();