Add a SplitRenderElements type, with useful methods

`(w_elements, p_elements)` tuples are used in a bunch of places. A
struct with named fields is generally an improvement just due to the
fact the order is non-obvious.

But we can also add methods. In particular,
`extend_from_workspace_elements` abstracts out some of the more
redundant code in `workspace_elements`.

It would be nice to avoid allocation everywhere, but iterators would
complicate lifetimes, run into issues with needing multiple mutable
borrows to things like the `Renderer`, and be awkward in certain
functions without generator syntax. In any case, cosmic-comp already
relies on allocating vectors here.

If this abstraction is commonly useful in compositors, perhaps it could
be moved to Smithay.

src/shell/grabs/moving.rs

@@ -4,7 +4,7 @@ use crate::{
     backend::render::{
         cursor::{CursorShape, CursorState},
         element::AsGlowRenderer,
-        BackdropShader, IndicatorShader, Key, Usage,
+        BackdropShader, IndicatorShader, Key, SplitRenderElements, Usage,
     },
     shell::{
         element::{
@@ -182,7 +182,10 @@ impl MoveGrabState {
             _ => vec![],
         };
 
-        let (window_elements, popup_elements) = self
+        let SplitRenderElements {
+            w_elements,
+            p_elements,
+        } = self
             .window
             .split_render_elements::<R, CosmicMappedRenderElement<R>>(
                 renderer,
@@ -202,9 +205,9 @@ impl MoveGrabState {
                     1.0,
                 )
             })
-            .chain(popup_elements)
+            .chain(p_elements)
             .chain(focus_element)
-            .chain(window_elements.into_iter().map(|elem| match elem {
+            .chain(w_elements.into_iter().map(|elem| match elem {
                 CosmicMappedRenderElement::Stack(stack) => {
                     CosmicMappedRenderElement::GrabbedStack(
                         RescaleRenderElement::from_element(