Define a RendererRef enum, referencing multi or glow renderer

Avoids a little duplication (matching two variants, instead of three
backends).

The behavior, including errors and panics, should be unchanged for now.
Performance should also not be impacted.

This should help for adding llvmpipe rendering without a device node on
the kms backend, or for adding a variant for pixman.

src/state.rs

@@ -1,7 +1,12 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 use crate::{
-    backend::{kms::KmsState, winit::WinitState, x11::X11State},
+    backend::{
+        kms::KmsState,
+        render::{GlMultiError, RendererRef},
+        winit::WinitState,
+        x11::X11State,
+    },
     config::{Config, OutputConfig, OutputState},
     input::gestures::GestureState,
     shell::{grabs::SeatMoveGrabState, CosmicSurface, SeatExt, Shell},
@@ -26,7 +31,7 @@ use once_cell::sync::Lazy;
 use rust_embed::RustEmbed;
 use smithay::{
     backend::{
-        allocator::dmabuf::Dmabuf,
+        allocator::{dmabuf::Dmabuf, Fourcc},
         drm::DrmNode,
         renderer::{
             element::{
@@ -380,6 +385,50 @@ impl BackendData {
         };
         Ok(None)
     }
+
+    /// Get an offscreen renderer for screen capture / screenshot rendering
+    ///
+    /// `kms_node_cb` callback use used to determine nodes to render with when using kms backend.
+    /// Panics if this returns `None`.
+    pub fn offscreen_renderer<N: Into<KmsNodes>, F: FnOnce(&mut KmsState) -> Option<N>>(
+        &mut self,
+        kms_node_cb: F,
+    ) -> Result<RendererRef, GlMultiError> {
+        match self {
+            BackendData::Kms(kms) => {
+                let KmsNodes {
+                    render_node,
+                    target_node,
+                    copy_format,
+                } = kms_node_cb(kms).expect("No Software Rendering").into();
+                Ok(RendererRef::GlMulti(kms.api.renderer(
+                    &render_node,
+                    &target_node,
+                    copy_format,
+                )?))
+            }
+            BackendData::Winit(winit) => Ok(RendererRef::Glow(winit.backend.renderer())),
+            BackendData::X11(x11) => Ok(RendererRef::Glow(&mut x11.renderer)),
+            _ => unreachable!("No backend set when getting offscreen renderer"),
+        }
+    }
+}
+
+pub struct KmsNodes {
+    pub render_node: DrmNode,
+    pub target_node: DrmNode,
+    pub copy_format: Fourcc,
+}
+
+impl From<DrmNode> for KmsNodes {
+    fn from(node: DrmNode) -> Self {
+        KmsNodes {
+            render_node: node,
+            target_node: node,
+            // Ignored if render == target
+            copy_format: Fourcc::Abgr8888,
+        }
+    }
 }
 
 pub fn client_has_no_security_context(client: &Client) -> bool {