We now just look for SDL2/SDL.h (the regular location) and SDL.h (when
the user provides their own location for this header).

The scene now owns its renderer, so there is no need to keep a list
of existing renderers and to call Renderer::Get().

This was not a very smart class. We replace it with a bidirectional map.

Sometimes libEGL.so can be available (provided as a symlink
by Debian diversion packages) while EGL/egl.h isn’t.

Let’s be honest, I’m never gonna use it in its current form.

If resolution is set to (0,0) then a fullscreen app is created.

The feature will still be accessible from a uniform in the shader, but it
is now disabled by default.