use env_logger in all examples (#2934)

examples/custom_3d_glow/src/main.rs

@@ -7,6 +7,7 @@ use egui::mutex::Mutex;
 use std::sync::Arc;
 
 fn main() -> Result<(), eframe::Error> {
+    env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
         initial_window_size: Some(egui::vec2(350.0, 380.0)),
         multisampling: 4,