fix memory leak (and crash under debugging) by not trying to unload textures on a non-GL thread

Program.cs

@@ -212,10 +212,10 @@ public class Photo {
     image = tmp;
   }
 
-  public async void UnloadAsync() {
+  public async void Unload() {
     Loaded = false;
     if (texture != placeholder) {
-      await Task.Run( () => { texture.Dispose(); });
+      texture.Dispose();
       texture = placeholder;
     }
   }
@@ -812,7 +812,7 @@ public class Game : GameWindow {
     int minLoadedImage = Math.Max(0, photoIndex - 20);
     int maxLoadedImage = Math.Min(photoIndex + 20, photos.Count - 1);
     // First, unload images that haven't been touched in a while.
-    // FIXME: also cancel any of these if they still have an in-progress loading task -- I suspect this is the source of a memory leak.
+    // FIXME: also cancel any of these if they still have an in-progress loading task?
     // FIXME: keep around thumbnail-sized textures?
     while (loadedImages.Count > 60) {
       long earliestTime = long.MaxValue;
@@ -825,7 +825,8 @@ public class Game : GameWindow {
       }
       if (earliest != null) {
         Console.WriteLine($"loadedImages.Count: {loadedImages.Count}, evicting {earliest.Filename} @ {earliestTime}");
-        earliest.UnloadAsync();
+        // TODO: we have to free textures on the GL thread, but could we do that async'ly to keep the UI responsive?
+        earliest.Unload();
         loadedImages.Remove(earliest);
       }
     }