Add player sprite textures back into the engine as a fallback (#8823)

Add player sprite textures back into the engine as a fallback (#8823)

'player.png' and 'player_back.png' are hardcoded into the engine as
the default appearence of the player object, so the textures should
be present in the engine.
This results in a functional fallback sprite player for when a game
has no player appearence related code.