Introduce vastly improved rendering of the game

This does proper rescaling of game resources, which **especially**
makes a difference when full-screen. No more jagged anti-aliased
TrueType fonts, w00t!
  This puts us back on mark from the long-ago SDL1 branch in terms of
our rescaling rendering capability.

* config.json: Change RENDER_SCALE_QUALITY default to 'nearest'; this
improves the rendering of card faces by not anti-aliasing the edges
of the pixels.

* Game.cpp: Remove 'loading screen' rect / text; this hasn't been used
since the move over to SDL2.

* config.json: Remove INFO_FONTFACE && INFO_SMALL_FONTFACE cfg options;
these are no longer used (libRocket only supports TrueType fonts).

* config.json: Remove MENU_ELEMENTS cfg options; these are no longer
used -- these are now loaded through the libRocket interface.