Better graphics backend fallback

This attempts the following in order:
- wgpu with user-selected backend
- wgpu with automatic backend
- glow (fallback OpenGL backend)

This should eliminate most issues
where objdiff fails to launch.

objdiff-gui/src/views/graphics.rs

@@ -36,7 +36,7 @@ pub enum GraphicsBackend {
     OpenGL,
 }
 
-#[derive(Debug, Default, serde::Deserialize, serde::Serialize)]
+#[derive(Clone, Debug, Default, serde::Deserialize, serde::Serialize)]
 pub struct GraphicsConfig {
     #[serde(default)]
     pub desired_backend: GraphicsBackend,