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  │ ║
+║ └────────┴───────┴───────┘ ║
+╚════════════════════════════╝

src/Spectre.Console.Tests/Unit/TableTests.cs

@@ -476,5 +476,26 @@ namespace Spectre.Console.Tests.Unit
             // Then
             return Verifier.Verify(console.Output);
         }
+
+        [Fact]
+        [Expectation("Render_Fold")]
+        public Task Should_Render_With_Folded_Text_Table_Correctly()
+        {
+            // Given
+            var console = new TestConsole().Width(30);
+            var table = new Table();
+            table.AddColumns("Foo", "Bar", "Baz");
+            table.AddRow("Qux With A Long Description", "Corgi", "Waldo");
+            table.AddRow("Grault", "Garply", "Fred On A Long Long Walk");
+
+            var panel = new Panel(table);
+            panel.Border = BoxBorder.Double;
+
+            // When
+            console.Write(panel);
+
+            // Then
+            return Verifier.Verify(console.Output);
+        }
     }
 }

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>();