Make it possible to inject additional image into image loader cache

Originally created by @jhutar in #232

I have images dirt.png and bush.png and with this I can create temporary image on the fly when starting the game (so my actor do not need to draw two overlapping images when running the game):

import pygame.image
img_dirt = pygame.image.load('images/dirt.png').convert_alpha()
img_bush = pygame.image.load('images/bush_small.png').convert_alpha()
img_dirt.blit(img_bush, (0,0))
images.add('tmp_dirt_with_bush', img_dirt)
my_actor.image = 'tmp_dirt_with_bush'

Please let me know if it makes sense.