draw black background for thumbnail boxes

Program.cs

@@ -355,6 +355,7 @@ public class UiGeometry {
   private static CameraInfo activeCamera = CameraInfo.CANON_EOS_R6M2;
 
   public readonly Vector2i WindowSize;
+  public readonly Box2i ThumbnailBox;
   public readonly List<Box2i> ThumbnailBoxes = new();
   public readonly Box2i PhotoBox;
   public readonly Box2i StatusBox;
@@ -376,6 +377,7 @@ public class UiGeometry {
     int statusBoxPadding = 4;
     PhotoBox = new Box2i(0, 0, WindowSize.X - thumbnailWidth, WindowSize.Y - statusBoxHeight - statusBoxPadding);
     StatusBox = new Box2i(0, WindowSize.Y - statusBoxHeight, WindowSize.X - thumbnailWidth, WindowSize.Y);
+    ThumbnailBox = new Box2i(ThumbnailBoxes[0].Min.X, ThumbnailBoxes[0].Min.Y, WindowSize.X, WindowSize.Y);
   }
 }
 
@@ -620,18 +622,12 @@ public class Game : GameWindow {
     }
 
     Vector2i renderSize = (Vector2i) (((Vector2) active.Size) * scale);
-    // FIXME: clip by the actual PhotoBox size -- this bleeds over UI elements when zoomed in.
-    // This doesn't work -- boo.
-    // if (renderSize.X > geometry.PhotoBox.Size.X) {
-    //   renderSize.X = geometry.PhotoBox.Size.X;
-    // }
-    // if (renderSize.Y > geometry.PhotoBox.Size.Y) {
-    //   renderSize.Y = geometry.PhotoBox.Size.Y;
-    // }
-
     Vector2i center = (Vector2i) geometry.PhotoBox.Center;
     Box2i photoBox = Util.MakeBox(center.X - renderSize.X / 2, center.Y - renderSize.Y / 2, renderSize.X, renderSize.Y);
     DrawTexture(active, photoBox); 
+
+    // Draw thumbnail boxes.
+    DrawFilledBox(geometry.ThumbnailBox, Color4.Black);
     for (int i = 0; i < photos.Count && i < geometry.ThumbnailBoxes.Count(); i++) {
       Box2i box = geometry.ThumbnailBoxes[i];
       DrawTexture(photos[i].Texture(), box);