Fix bitmaptools.dither() into 1-bit dest

[FoamyGuy]

This change fixes an issue that causes a hard fault when attempting to dither into a 1-bit destination. Here is reproducer code:

import displayio
import bitmaptools


def check(width, height):
    src = displayio.Bitmap(width, height, 65536)
    # Horizontal luminance ramp: nontrivial dither pattern in every byte.
    for x in range(width):
        v = x * 255 // max(width - 1, 1)
        rgb565 = ((v >> 3) << 11) | ((v >> 2) << 5) | (v >> 3)
        for y in range(height):
            src[x, y] = rgb565

    ref = displayio.Bitmap(width, height, 65536)
    bitmaptools.dither(ref, src, displayio.Colorspace.RGB565,
                       bitmaptools.DitherAlgorithm.FloydStenberg)

    dst = displayio.Bitmap(width, height, 2)
    # Broken firmware dies on this call.
    bitmaptools.dither(dst, src, displayio.Colorspace.RGB565,
                       bitmaptools.DitherAlgorithm.FloydStenberg)

    mismatches = 0
    first = None
    for y in range(height):
        for x in range(width):
            if (ref[x, y] != 0) != (dst[x, y] != 0):
                mismatches += 1
                if first is None:
                    first = (x, y)
    if mismatches:
        return "FAIL: %d/%d pixels differ, first at %s" % (
            mismatches, width * height, first)
    return "PASS"


for size in ((320, 4), (64, 4), (50, 3), (17, 3), (8, 2), (1, 1)):
    print("%dx%d:" % size, check(*size))
print("done")

On a very recent main build from S3 it hard faults:

Adafruit CircuitPython 11.0.0-alpha.0-7-g8f94f74a7c on 2026-09-23; Adafruit Fruit Jam with rp2350b
>>> 
>>> import dither_issue

[16:16:35.130] Disconnected
[16:16:36.130] Connected
Auto-reload be off.
Runnin' in safe mode! Nay runnin' saved code.

You are in safe mode because:
CircuitPython core code crashed hard. Whoops!
Hard fault: memory access or instruction error.
Please file an issue with your program at github.com/adafruit/circuitpython/issues.
Press reset to exit safe mode.

Press any key to enter the REPL. Use CTRL-D to reload.

With this fix in place it runs successfully

>>> import dither_issue
320x4: PASS
64x4: PASS
50x3: PASS
17x3: PASS
8x2: PASS
1x1: PASS
done

[dhalbert]

Makes sense - thanks!