Add support for fake input in asciicast recordings

* Fixes a bug with `SelectionPrompt` and page size.
* Allow `IAnsiConsoleInput` to return `null`.

resources/scripts/Generator/Commands/AsciiCast/Samples/CanvasSample.cs

@@ -0,0 +1,29 @@
+using Spectre.Console;
+
+namespace Generator.Commands.Samples
+{
+    internal class CanvasSample : BaseSample
+    {
+        public override void Run(IAnsiConsole console)
+        {
+            var canvas = new Canvas(16, 16);
+
+            // Draw some shapes
+            for(var i = 0; i < canvas.Width; i++)
+            {
+                // Cross
+                canvas.SetPixel(i, i, Color.White);
+                canvas.SetPixel(canvas.Width - i - 1, i, Color.White);
+
+                // Border
+                canvas.SetPixel(i, 0, Color.Red);
+                canvas.SetPixel(0, i, Color.Green);
+                canvas.SetPixel(i, canvas.Height - 1, Color.Blue);
+                canvas.SetPixel(canvas.Width - 1, i, Color.Yellow);
+            }
+
+            // Render the canvas
+            console.Write(canvas);
+        }
+    }
+}