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.Tests/Expectations/Widgets/Table/Render_Fold.Output.verified.txt

@@ -0,0 +1,16 @@
+╔════════════════════════════╗
+║ ┌────────┬───────┬───────┐ ║
+║ │ Foo    │ Bar   │ Baz   │ ║
+║ ├────────┼───────┼───────┤ ║
+║ │ Qux    │ Corgi │ Waldo │ ║
+║ │ With A │       │       │ ║
+║ │ Long   │       │       │ ║
+║ │ Descri │       │       │ ║
+║ │ ption  │       │       │ ║
+║ │ Grault │ Garpl │ Fred  │ ║
+║ │        │ y     │ On A  │ ║
+║ │        │       │ Long  │ ║
+║ │        │       │ Long  │ ║
+║ │        │       │ Walk  │ ║
+║ └────────┴───────┴───────┘ ║
+╚════════════════════════════╝