gui: image_renderer: fix "no previous prototype for 'image_renderer_unregister'" warning ...

The patch fixes this compiler's warning:

  lib/gui/image_renderer.c:92:6: warning: no previous prototype for 'image_renderer_unregister' [-Wmissing-prototypes]
   void image_renderer_unregister(struct image_renderer *ir)
        ^

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

gui: introduce screen and surface to factorize and simplify code ...

Instead of passing hundreds of parameter, just pass the right structure.

struct screen represent the screen with a without double buffering.
struct surface represent the part of the screen we want to render.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

gui: move gui file to include/gui and lib/gui ...

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>