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;