example: an art with 4 frames and 3 layers

Art
|width, height, charset, palette: stuff that's written to / read from disk
|renderables: list of renderables using us
|vert_array, elem_array: geo array for all layers (changes on: resize, layer add/del)
|update lists: tiles of specific frame+layers whose array data we should update
|frames
   |0
      |delay: time before display next frame
      |uv_array, fg_color_array, bg_color array: arrays for all layers of this frame
	  |                            (changes on: tile edit, art resize, layer add/del)
      |layers
         |0
            |z: z depth for this layer
            |chars, fg_colors, bg_colors: data (lists of rows) for this layer
         |1
            |z
            |chars, fg_colors, bg_colors
         |2
            |z
            |chars, fg_colors, bg_colors
   |1
      |delay
      |uv_array, fg_color_array, bg_color array
      |layers
         |0
            |z
            |chars, fg_colors, bg_colors
         |1
            |z
            |chars, fg_colors, bg_colors
         |2
            |z
            |chars, fg_colors, bg_colors
   |2
      |delay
      |uv_array, fg_color_array, bg_color array
      |layers
         |0
            |z
            |chars, fg_colors, bg_colors
         |1
            |z
            |chars, fg_colors, bg_colors
         |2
            |z
            |chars, fg_colors, bg_colors
   |3
      |delay
	  |uv_array, fg_color_array, bg_color array
      |layers
         |0
            |z
            |chars, fg_colors, bg_colors
         |1
            |z
            |chars, fg_colors, bg_colors
         |2
            |z
            |chars, fg_colors, bg_colors