Add virtual destructor to Renderable class

Fix following compiler warnings:

source/code/LightsOutGame.cpp: In destructor ‘LightsOutGame::~LightsOutGame()’:
source/code/LightsOutGame.cpp:118:17: warning: deleting object of polymorphic class type ‘Light’ which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]
    delete tile->object;

source/code/LightsOutGame.cpp: In member function ‘virtual bool LightsOutGame::paint(SDL_Surface&, unsigned int, unsigned int) const’:
source/code/LightsOutGame.cpp:408:11: warning: deleting object of polymorphic class type ‘Light’ which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]
    delete l;

source/code/LightsOutGameManager.cpp: In member function ‘virtual void LightsOutGameManager::run()’:
source/code/LightsOutGameManager.cpp:252:17: warning: deleting object of polymorphic class type ‘LightsOutGame’ which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]
    delete this->game;
Return proper type from executor

Fix following compiler error:

source/code/Thread.cpp: In function ‘int executor(void*)’:
source/code/Thread.cpp:32:9: warning: converting to non-pointer type ‘int’ from NULL [-Wconversion-null]
  return NULL;
Fix unsigned integer comparisons where safe