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

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

examples/custom_3d_glow/Cargo.toml

@@ -12,6 +12,4 @@ publish = false
 eframe = { path = "../../crates/eframe", features = [
     "__screenshot", # __screenshot is so we can dump a screenshot using EFRAME_SCREENSHOT_TO
 ] }
-egui_glow = { path = "../../crates/egui_glow" }
 env_logger = "0.10"
-glow = "0.12"

examples/custom_3d_glow/src/main.rs

@@ -1,7 +1,7 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 #![allow(unsafe_code)]
 
-use eframe::egui;
+use eframe::{egui, egui_glow, glow};
 
 use egui::mutex::Mutex;
 use std::sync::Arc;

examples/test_inline_glow_paint/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "test_inline_glow_paint"
+version = "0.1.0"
+authors = ["Emil Ernerfeldt <emil.ernerfeldt@gmail.com>"]
+license = "MIT OR Apache-2.0"
+edition = "2021"
+rust-version = "1.72"
+publish = false
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+eframe = { path = "../../crates/eframe" }
+env_logger = "0.10"

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.");
+        });
+    }
+}