nom::Renderer: Introduce new features

* Methods: size, bounds, set_bounds
Introduce new convenience typedefs

* Also, relocate existing SDL pointer types from Window, Renderer, ...
to SDL_helpers.hpp
nom::Window: Fix save_screenshot method

* Re-implemented to support SDL2 accelerated rendering targets; a SDL1
window surface cannot be used on accelerated framebuffers.

  - nom::Renderer
    * Introduce pixels method

  - nom::Image
    * Introduce new initialize method; SDL_CreateRGBSurfaceFrom wrapper

  - examples/app
    * Introduce F1 key binding for taking screenshots of the focused
    window
nom::GrayFrame: Fix minor rendering bug