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/backend/render/mod.rs

@@ -53,6 +53,7 @@ use smithay::{
                 element::PixelShaderElement, GlesError, GlesPixelProgram, GlesRenderer, Uniform,
                 UniformName, UniformType,
             },
+            glow::GlowRenderer,
             multigpu::{Error as MultiError, MultiFrame, MultiRenderer},
             sync::SyncPoint,
             Bind, Blit, ExportMem, ImportAll, ImportMem, Offscreen, Renderer, TextureFilter,
@@ -86,6 +87,29 @@ pub type GlMultiFrame<'a, 'frame> =
     MultiFrame<'a, 'a, 'frame, GbmGlowBackend<DrmDeviceFd>, GbmGlowBackend<DrmDeviceFd>>;
 pub type GlMultiError = MultiError<GbmGlowBackend<DrmDeviceFd>, GbmGlowBackend<DrmDeviceFd>>;
 
+pub enum RendererRef<'a> {
+    Glow(&'a mut GlowRenderer),
+    GlMulti(GlMultiRenderer<'a>),
+}
+
+impl<'a> AsRef<GlowRenderer> for RendererRef<'a> {
+    fn as_ref(&self) -> &GlowRenderer {
+        match self {
+            Self::Glow(renderer) => renderer,
+            Self::GlMulti(renderer) => renderer.as_ref(),
+        }
+    }
+}
+
+impl<'a> AsMut<GlowRenderer> for RendererRef<'a> {
+    fn as_mut(&mut self) -> &mut GlowRenderer {
+        match self {
+            Self::Glow(renderer) => renderer,
+            Self::GlMulti(renderer) => renderer.as_mut(),
+        }
+    }
+}
+
 pub static CLEAR_COLOR: [f32; 4] = [0.153, 0.161, 0.165, 1.0];
 pub static OUTLINE_SHADER: &str = include_str!("./shaders/rounded_outline.frag");
 pub static RECTANGLE_SHADER: &str = include_str!("./shaders/rounded_rectangle.frag");