fix: Fix iTerm2's title with image support enabled

- Fix: Force support support only for the selected image format.

  If support is forced for `KittyImage`, then `UrwidImageScreen` will
  emit kitty graphics control sequences to delete images upon starting
  and stoping the screen.

iTerm2 (on Mac OS) doesn't eat up APCs (which kitty graphics control
sequences are) as it should, instead it writes them to the screen
and/or to its title bar.

toot/tui/images.py

@@ -28,7 +28,6 @@ try:
         global _ImageCls
 
         if not _ImageCls:
-            BaseImage.forced_support = True
             _ImageCls = (
                 KittyImage
                 if image_format == 'kitty'
@@ -36,6 +35,7 @@ try:
                 if image_format == 'iterm'
                 else BlockImage
             )
+            _ImageCls.forced_support = True
 
         return _ImageCls(image)