This gives us a reasonably cross-platform way to to build options. Works
well with LolRemez, where we no longer depend on GUI libraries.

Rename config-build.xml to build.config, move it to the root directory,
and add MSBuild flags for SDL, FFmpeg (unused yet) and OpenGL.

This flag disables GL, GLES, but also Glew and EGL libraries.

When built as a subproject, we need the master to decide what features
to activate or not. This could lead to reduced dependences.

This patch also allows to build without imlib2.

Linux-only for now. This will be necessary for projects that do not
actually use the rendering engine, such as LolRemez.

This will let us detect problems with unused return values. For instance,
if you don’t remember that normalize(q) returns a pointer and expect it
to modify the object, you’ll get this warning:

warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
    quat q(1,2,3,4);  normalize(q);
                      ^~~~~~~~~ ~

From now on, msg::debug will not display anything, unless this is a debug
build, or the LOL_DEBUG environment variable is set.

By adding a bias we ensure that 1.999… will be displayed as 2.0 as long as we
do not ask for too many digits, and that rounding is reasonable for most values.
Note that our need for proper rounding is not as high as for IEEE floats since
we have an insane amount of digits at our disposal.

Also we now get rid of trailing zeroes when printing reals.

It will now be available at https://github.com/samhocevar/lolremez

return 1 added after sam remark

Moved ImageLoader to ResourceLoader, so that loading is extensible.
i.e. here: Zed image loader now loads up a tileset instead of being a
hacked image loader.
Goal is to support more funky stuff (wad, pak .....)

imGui docking extension tweak
tileset new getters