Updated OnDrawingContent methods across multiple classes to use non-nullable DrawContext parameters, improving type safety.

Refactored `IDriver` interface:
- Updated `SetScreenSize` to remove `NotSupportedException`.
- Clarified `Refresh` method as internal and updated documentation for `SizeChanged` event.

Examples/UICatalog/Scenarios/Images.cs

@@ -631,7 +631,7 @@ public class Images : Scenario
         public Image<Rgba32> FullResImage;
         private Image<Rgba32> _matchSize;
 
-        protected override bool OnDrawingContent (DrawContext? context)
+        protected override bool OnDrawingContent (DrawContext context)
         {
             if (FullResImage == null)
             {
@@ -708,7 +708,7 @@ public class Images : Scenario
             return (columns, rows);
         }
 
-        protected override bool OnDrawingContent (DrawContext? context)
+        protected override bool OnDrawingContent (DrawContext context)
         {
             if (_palette == null || _palette.Count == 0)
             {

Examples/UICatalog/Scenarios/LineDrawing.cs

@@ -1,7 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
+using System.Text;
 
 namespace UICatalog.Scenarios;
 
@@ -273,7 +270,7 @@ public class DrawingArea : View
     public ITool CurrentTool { get; set; } = new DrawLineTool ();
     public DrawingArea () { AddLayer (); }
 
-    protected override bool OnDrawingContent (DrawContext? context)
+    protected override bool OnDrawingContent (DrawContext context)
     {
         foreach (LineCanvas canvas in Layers)
         {
@@ -380,7 +377,7 @@ public class AttributeView : View
     }
 
     /// <inheritdoc/>
-    protected override bool OnDrawingContent (DrawContext? context)
+    protected override bool OnDrawingContent (DrawContext context)
     {
         Color fg = Value.Foreground;
         Color bg = Value.Background;

Examples/UICatalog/Scenarios/Snake.cs

@@ -322,7 +322,7 @@ public class Snake : Scenario
 
         private SnakeState State { get; }
 
-        protected override bool OnDrawingContent (DrawContext? context)
+        protected override bool OnDrawingContent (DrawContext context)
         {
             SetAttribute (white);
             ClearViewport ();

Examples/UICatalog/Scenarios/TextEffectsScenario.cs

@@ -109,7 +109,7 @@ internal class GradientsView : View
     private const int LABEL_HEIGHT = 1;
     private const int GRADIENT_WITH_LABEL_HEIGHT = GRADIENT_HEIGHT + LABEL_HEIGHT + 1; // +1 for spacing
 
-    protected override bool OnDrawingContent (DrawContext? context)
+    protected override bool OnDrawingContent (DrawContext context)
     {
         DrawTopLineGradient (Viewport);