Add code to generate test picture

def _append_pixel(red: int, green: int, blue: int, pixels: list[int]) -> None:

    """

    Append a pixel with the given red, green and blue parts to the list of pixels.

    The pixel is represented as 16-bit, like this (5 bit for red, 6 bit for green, 5 bit for blue):

    RRRRRGGG|GGGBBBBB

    Two integers are appended to the list.

    - Bit order: Big endian (i.e. within one byte, the highest valued bit comes first; this is the normal way of writing down a number)

    - Byte order: Little endin (i.e. in a sequence of bytes, the highest valued byte comes last; this is the weird way of writing down a sequence)

    """

    high_byte = red << 3 | green >> 3  # RRRRRGGG

    low_byte = (green << 5) & 0xFF | blue  # GGGBBBBB

    pixels.append(low_byte)

    pixels.append(high_byte)

def get_test_picture(width: int, height: int) -> bytes:

    """

    Generate a test picture with a color gradient and return it.

    """

    picture = []

    for y in range(height):

        for x in range(width):

            b = 10

            g = int(x * 31.0 / 480.0)

            r = int(x * 31.0 / 480)

            _append_pixel(red=r, green=g, blue=b, pixels=picture)

    return bytes(picture)