displayio: added Mapping to bits_per_value getter

[gaweng]

fixed #4424

[FoamyGuy]

This looks good to me.

I tested it successfully with this code on a Matrix Portal S3:

import random
import time
import board
import displayio
from adafruit_matrixportal.matrix import Matrix
import bitmaptools

main_group = displayio.Group()

matrix = Matrix()
display = matrix.display

display.show(main_group)

bitmap = displayio.Bitmap(matrix.display.width, matrix.display.height, 257)
print(f"{bitmap.width}, {bitmap.height}")
print(f"bits per value: {bitmap.bits_per_value}")

p = displayio.Palette(3)
p[0] = 0x000000
p[1] = 0x00ff00

bitmaptools.draw_line(bitmap, 0, 0, bitmap.width-1, bitmap.height, 1)
tg = displayio.TileGrid(bitmap=bitmap, pixel_shader=p)
main_group.append(tg)

while True:
    pass

[tannewt]

Thank you!