Image Manipulation: 255 Shades of Grey (Part 2)

Images can be described as 3D arrays.

// This image has only one white pixel:

[
  [[255, 255, 255]]
]

// This one is a 2 by 2 black image:

[
  [[0, 0, 0], [0, 0, 0]],
  [[0, 0, 0], [0, 0, 0]]
]

Your task is to create a function that takes a 3D array representation of an image and returns the grayscale version of that.

Examples

grayscale([
  [[0, 0, 0], [0, 0, 157]],
  [[1, 100, 0], [0, 10, 0]]
]) ➞ [
  [[0, 0, 0], [52, 52, 52]],
  [[34, 34, 34], [3, 3, 3]]
]

Notes

• A valid RGB value ranges from 0 to 255 (inclusive).
• If the given array contains out of bound values, convert them to the nearest valid one.
• Previous challenge in this series: Image Manipulation: Invert (Part 1)
• Grayscaling an image is adjusting to have the same amount of (Red, Green, Blue) from their combined average to produce different shades of gray.