eframe glow backend: Clear render target before calling App::update (#3665)

* Closes https://github.com/emilk/egui/issues/3659

examples/test_inline_glow_paint/src/main.rs

@@ -0,0 +1,40 @@
+// Test that we can paint to the screen using glow directly.
+
+use eframe::egui;
+use eframe::glow;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
+    let options = eframe::NativeOptions {
+        renderer: eframe::Renderer::Glow,
+        ..Default::default()
+    };
+    eframe::run_native(
+        "My test app",
+        options,
+        Box::new(|_cc| Box::<MyTestApp>::default()),
+    )?;
+    Ok(())
+}
+
+#[derive(Default)]
+struct MyTestApp {}
+
+impl eframe::App for MyTestApp {
+    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
+        use glow::HasContext as _;
+        let gl = frame.gl().unwrap();
+
+        #[allow(unsafe_code)]
+        unsafe {
+            gl.disable(glow::SCISSOR_TEST);
+            gl.viewport(0, 0, 100, 100);
+            gl.clear_color(1.0, 0.0, 1.0, 1.0); // purple
+            gl.clear(glow::COLOR_BUFFER_BIT);
+        }
+
+        egui::Window::new("Floating Window").show(ctx, |ui| {
+            ui.label("The background should be purple.");
+        });
+    }
+}