Screen is a RenderTarget

[asny]

Implement Screen as a special RenderTarget, so Screen::new() should return a RenderTarget.
Implement read_color and read_depth functionality for standard render targets.

See this branch.

• render_pass should take render target as input
• Makes it possible to do the deferred render call in one method which takes a render target as input
• Should be possible to get the screen render target from the window (Prerequisite for Run examples on CI #199 )

[asny]

Deferred rendering improvements are continued in #237, otherwise done.