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/Cargo.toml

@@ -18,7 +18,7 @@ name = "objdiff"
 path = "src/main.rs"
 
 [features]
-default = ["wgpu", "wsl"]
+default = ["glow", "wgpu", "wsl"]
 glow = ["eframe/glow"]
 wgpu = ["eframe/wgpu", "dep:wgpu"]
 wsl = []