(1)-'Arena' class now handles arenas with and without background images appropriately.  (2)-'arena_nt' now takes values for 'left_wall_x', 'right_wall_x', and 'floor_y', 'Arena' class changed accordingly.  (3)-'out_of_arena_fix' now works according to 'play_area_rect' size instead of using hard-coded values.  (4)-Added debug method '_draw_play_area_debug_border' to draw rectangles outlining the play_area, walls, and floor rects.
(1)-Created a sprite for player 2 by simply changing red to blue since this >>> STILL HADN'T BEEN DONE (I'm looking at you, Peter) <<<.  (2)-'_setup_player_sprites' is now a subfunction of new '_setup_players' function and takes a spritesheet filename parameter; this avoids code duplication for the second player's sprite setup.  (3)-'_draw_players' now has a subfunction, '_draw_player', that takes a player parameter; this avoids code duplication for player 2.  (4)-'_draw_players_debug' now has a subfunction, '_draw_player_debug', that takes a few parameters; this avoids code duplication for player2.  (5)-Reordering of some methods within 'GameLoop.__init__'.  (6)-'Player' class now takes a sprite parameter.