Add glow::Context to epi::Frame (#1425)

This can be used, for instance, to:

* Render things to offscreen buffers.
* Read the pixel buffer from the previous frame (glow::Context::read_pixels).
* Render things behind the egui windows.

egui_demo_lib/src/backend_panel.rs

@@ -12,7 +12,7 @@ enum RunMode {
     /// For instance, a GUI for a thermostat need to repaint each time the temperature changes.
     /// To ensure the UI is up to date you need to call `egui::Context::request_repaint()` each
     /// time such an event happens. You can also chose to call `request_repaint()` once every second
-    /// or after every single frame - this is called `Continuous` mode,
+    /// or after every single frame - this is called "Continuous" mode,
     /// and for games and interactive tools that need repainting every frame anyway, this should be the default.
     Reactive,