Add glow::Context to epi::Frame

[emilk]

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.

Note that all egui painting is deferred to after the call to App::update (egui only collects egui::Shapes and then eframe paints them all in one go later on).

[BernardBorio]

Hi, I'm trying to export the painter_layer content as an image, and I've seen you made this function to read the pixels but I have no idea how to use it, can you explain me a bit? It could be great 🙏🏼