Clear the stale bitmap signature when preserving an internal copy

crates/ui-egui/src/app/clipboard.rs

@@ -121,6 +121,7 @@ impl PhotocraftApp {
         let image = image.filter(|(w, h, bytes)| *w > 0 && *h > 0 && bytes.len() == *w as usize * *h as usize * 4);
         let Some((w, h, bytes)) = image else {
             if self.clip_unmirrored && !self.clip_external && self.session.clipboard.is_some() {
+                self.os_clip_sig = None;
                 return Ok(false);
             }
             self.os_clip_svg = None;