#8 Add ImageSheet system

[Reylak]

We plan to change the way Sprites are created (more than [#7]): instead of giving them an image, we give a table of frames called a SpriteSheet.

Quoting Lolman (Jacen.Guardian):

I think what follows:

• Sprite should work with a SpriteSheet (or Anim, choose the name you want) object;
• SpriteSheet would be in fact a table of Image which would have a
  2 ctor;
• the first ctor takes one filename, the width and the height of one frame as its arguments;
• the other ctor takes a list of files which are individually the frames (1 file = 1 frame);
• then we can access the resulting table of Image using SpriteSheet[n];
• finally, the sprite ctor becomes "Sprite.new(tableOfImage, delay)", so there are many possibilities: "Sprite.new(SpriteSheet1, 50)", "Sprite.new({ SprSheet2[1], SprSheet1[3], SpriteAnim3[1] }, 100)", etc.;
• finally the Images are outside the Sprite, editable, and we can even draw them directly (this isn't a replacement of drawFrame but could be): "screen.blit(SCREEN_UP, 20, 20, SprSheet1[1])". We can also use the same frame of a spritesheet for many sprites.

Instead of making two constructors, we only make one that is able to handle both calls (by checking the types of its arguments).

[Reylak]

Modification: this system should be generic and could be used for Maps too.

We would have a function to load and split the image file, then we can create a new Sprite or Map from it.