lib/sdl: intro services to get the X11 window handle

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/sdl: minor whitespace cleanup

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: intro of the egl module

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: intro of the x11 module

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: intro of the glesv2 module

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples: adds the hello_triangle example using glesv2, x11 and egl

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: skip example opengles2_hello_triangle for nitg-s

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge remote-tracking branch 'alexis/glesv2'

...also adds an usage example.

To do next:
* Adapt the pkgconfig annotation to accept no arguments
* Implement mnit::Display feature entierly in Nit using these modules
* Add more examples
* Reify some concepts in glesv2