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.

This branch seems pretty stable, and it lets us get rid of some
legacy code about extensions.

We used to have lol::map::empty() to empty maps, but in std::map
this method is called clear(). It sounds safe to harmonise between
our types and have lol::array::clear() too.

bullet3, pegtl: update upstream branches.
imgui: add missing files in “make dist”.

The only reason we had to keep this was for emscripten builds. We can
now use SDL v2 using “emconfigure ./configure CFLAGS="-s USE_SDL=2” and
a few additional flags.

SDL_Mixer v2 is not available in emscripten packages but either it will
be added at some time, or I will get rid of it altogether. An alternate
solution could be this code snippet I found:

    cd SDL2_mixer
    emconfigure ./configure --prefix=$(pwd)/dist --enable-music-ogg --disable-music-ogg-shared CFLAGS="-s USE_VORBIS=1"
    emmake make install

Previous code relied on preprocessor check for INT_MAX != PTRDIFF_MAX but
it is a lot more reliable to use std::enable_if instead.