Apply screencopy transform to subsurfaces

Fixes https://github.com/pop-os/cosmic-workspaces-epoch/issues/17.

src/backend/mod.rs

@@ -47,6 +47,7 @@ pub struct CaptureImage {
     pub width: u32,
     pub height: u32,
     pub wl_buffer: SubsurfaceBuffer,
+    pub transform: wl_output::Transform,
     #[cfg(feature = "no-subsurfaces")]
     pub image: cosmic::widget::image::Handle,
 }