Not relying on GL version when asking for extensions, i.e. Mesa might
know glGetStringi() even if it reports OpenGL 2.1 only. Similarly in
shadingLanguageVersionStrings(), not all HW is capable of 4.3, but
GL_NUM_SHADING_LANGUAGE_VERSIONS might be supported on GL3 HW too (not
my case, though).
In next few commits AbstractFont will become plugin interface. Font
implementations are now in magnum-plugins repository. Removed all traces
of FreeType and HarfBuzz dependencies.
Similar to `glxinfo`, displays information about Magnum engine and
OpenGL capabilities. Currently for Unix/X11 only, as I don't have
windowless application implementation for other platforms.