Protect selected windows from screen capture

Add capture privacy rules for windows, transient children, overview previews and client cursors, with separate capture rendering and deferred-frame invalidation. Keep detached close snapshots out of captures after clients leave the registry.

Validated with release workspace tests, private compositor/EGL regressions, and an isolated release PipeWire capture test.

compositor/src/render/mod.rs

@@ -23,7 +23,7 @@ use overview::*;
 pub(crate) use resources::RenderResources;
 use scene::cursor_elements;
 pub(crate) use scene::{
-    animated_window_elements, frame_effect_metrics, layer_surfaces, output_elements, sampled_output_elements,
+    animated_window_elements, capture_output_elements, frame_effect_metrics, layer_surfaces, sampled_output_elements,
 };
 pub(crate) use shader::SharedPixelShaderElement;
 use smithay::backend::allocator::Fourcc;