Miscellany

	//1 inclusive 2 exclusive
	Picture crop(int x1, y1, x2, y2){
		uint width = x2 - x1;
		uint height = y2 -y1;
		ubyte[] data;
		
		uint current;
		for(uint y = y1; y < y2; y++){
			current = this.width * y + x1;
			data ~= this.data[current..current + width];
		}
		return new Picture(data, width, height);
	}