Transitions: fade starts from the outgoing picture (was an instant cut)

The transition started with no from-frame, so BlendFrame returned the new scene at once.
Start now requests a capture and the frame pump supplies its last output on the next tick.
Test: TransitionFadeTests. Docs: ai.md, TASKS.md.

Not pushed.

ytLive.Tests/TransitionFadeTests.cs

@@ -0,0 +1,26 @@
+using ytLive.Services;
+using Xunit;
+
+namespace ytLive.Tests;
+
+public class TransitionFadeTests
+{
+    [Fact]
+    public void Fade_Without_A_From_Frame_Requests_Capture_And_Blends_Once_Supplied()
+    {
+        var transition = new TransitionService();
+        var to = new VideoFrame(2, 2, Enumerable.Repeat((byte)255, 16).ToArray());
+        var from = new VideoFrame(2, 2, new byte[16]);
+
+        transition.Start(TransitionType.Fade, 300, null);
+        Assert.True(transition.CaptureRequested);
+
+        transition.SupplyFromFrame(from);
+        Assert.False(transition.CaptureRequested);
+
+        transition.Tick(150);
+        var blended = transition.BlendFrame(to);
+        Assert.NotSame(to, blended);
+        Assert.NotEqual((byte)255, blended.BgraPixels[0]);
+    }
+}