Automatically generate screenshots for all examples (#2379)

examples/custom_3d_glow/Cargo.toml

@@ -9,6 +9,8 @@ publish = false
 
 
 [dependencies]
-eframe = { path = "../../crates/eframe", features = ["glow"] }
+eframe = { path = "../../crates/eframe", features = [
+    "__screenshot", # __screenshot is so we can dump a ascreenshot using EFRAME_SCREENSHOT_TO
+] }
 egui_glow = { path = "../../crates/egui_glow" }
 glow = "0.11"

examples/custom_3d_glow/README.md

@@ -13,3 +13,5 @@ If you are content of having egui sit on top of a 3D background, take a look at:
 ```sh
 cargo run -p custom_3d_glow
 ```
+
+![](screenshot.png)