Keep track of RenderElementStates in offscreens

This both avoids sending frame callbacks to surfaces invisible on the offscreen
(fixing Firefox with subsurface compositing in the process), and fixes
searching for split popups during the resize animation.

niri-visual-tests/src/test_window.rs

@@ -6,6 +6,7 @@ use niri::layout::{
     ConfigureIntent, InteractiveResizeData, LayoutElement, LayoutElementRenderElement,
     LayoutElementRenderSnapshot,
 };
+use niri::render_helpers::offscreen::OffscreenData;
 use niri::render_helpers::renderer::NiriRenderer;
 use niri::render_helpers::solid_color::{SolidColorBuffer, SolidColorRenderElement};
 use niri::render_helpers::{RenderTarget, SplitElements};
@@ -214,7 +215,7 @@ impl LayoutElement for TestWindow {
 
     fn output_leave(&self, _output: &Output) {}
 
-    fn set_offscreen_element_id(&self, _id: Option<Id>) {}
+    fn set_offscreen_data(&self, _data: Option<OffscreenData>) {}
 
     fn set_activated(&mut self, _active: bool) {}