#10 Implement render-to-texture for in-game screens

[Gordon Tisher]

Implement render-to-texture so that we can use subclasses of gsgl::platform::display for in-game screens.

This involves creating a new node type (gsgl::scenegraph::screen, perhaps) that contains a gsgl::platform::display object that renders to a texture.

Before the main view is drawn, render all screens.