Quantise colour before writing (#1247)

closes
RaspberryPiFoundation/digital-editor-issues#871

Code used to test -
```
from sense_hat import SenseHat
from time import sleep

sense = SenseHat()

# --- Quantisation helper to match set_pixel ---
def quantise_rgb(r, g, b):
    r = (r >> 3) << 3
    g = (g >> 2) << 2
    b = (b >> 3) << 3
    return (r, g, b)

# --- Colours ---
colours = [
    (255, 255, 255),  # White
    (255, 0, 0),      # Red
    (0, 255, 0),      # Green
    (0, 0, 255),      # Blue
    (245, 245, 245),  # WhiteSmoke
    (220, 220, 220),  # Gainsboro
    (192, 192, 192),  # Silver
    (105, 105, 105),  # DimGray
    (169, 169, 169),  # DarkGray
    (0, 0, 0),        # Black
    (100, 149, 237),  # CornflowerBlue
    (70, 130, 180),   # SteelBlue
    (0, 0, 205),      # MediumBlue
    (25, 25, 112),    # MidnightBlue
    (0, 0, 139),      # DarkBlue
    (0, 191, 255),    # DeepSkyBlue
    (30, 144, 255),   # DodgerBlue
    (135, 206, 250),  # LightSkyBlue
    (0, 255, 255),    # Cyan
    (64, 224, 208),   # Turquoise
    (0, 128, 128),    # Teal
    (143, 188, 143),  # DarkSeaGreen
    (60, 179, 113),   # MediumSeaGreen
    (46, 139, 87),    # SeaGreen
    (0, 255, 127),    # SpringGreen
    (34, 139, 34),    # ForestGreen
    (50, 205, 50),    # LimeGreen
    (154, 205, 50),   # YellowGreen
    (128, 128, 0),    # Olive
    (85, 107, 47),    # DarkOliveGreen
    (240, 230, 140),  # Khaki
    (255, 255, 0),    # Yellow
    (255, 215, 0),    # Gold
    (184, 134, 11),   # DarkGoldenrod
    (139, 69, 19),    # SaddleBrown
    (210, 105, 30),   # Chocolate
    (255, 140, 0),    # DarkOrange
    (255, 69, 0),     # OrangeRed
    (178, 34, 34),    # Firebrick
    (255, 0, 0),      # Red
    (220, 20, 60),    # Crimson
    (255, 192, 203),  # Pink
    (255, 105, 180),  # HotPink
    (255, 20, 147),   # DeepPink
    (199, 21, 133),   # MediumVioletRed
    (153, 50, 204),   # DarkOrchid
    (148, 0, 211),    # DarkViolet
    (138, 43, 226),   # BlueViolet
    (75, 0, 130),     # Indigo
]

passed_count = 0
failed_count = 0

for c in colours:
    r, g, b = c
    sense.clear()
    sense.set_pixel(3, 3, r, g, b)

    actual = tuple(sense.get_pixel(3, 3))
    expected = quantise_rgb(r, g, b)  # match set_pixel quantization

    if actual == expected:
        print(f"✅ PASS: Requested {c} -> Quantised {expected} -> Actual {actual}")
        passed_count += 1
    else:
        print(f"❌ FAIL: Requested {c} -> Quantised {expected} -> Actual {actual}")
        failed_count += 1

    sleep(0.3)

sense.clear()

print(f"\n🎉 Tests complete! Passed: {passed_count}, Failed: {failed_count}")

```

Author: jamiebenstead

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## Unreleased
 
+### Changed
+
+- Changed `set_pixel` to quantise the colour before writing (#1247)
+
 ## [0.31.1] - 2025-09-19
 
 ### Added

public/shims/sense_hat/sense_hat_blob.py

@@ -1095,6 +1095,13 @@ def set_pixel(self, x, y, *args):
             if element > 255 or element < 0:
                 raise ValueError('Pixel elements must be between 0 and 255')
 
+        # 🔹 Quantise the colour before writing
+        r, g, b = pixel
+        r = (r >> 3) << 3
+        g = (g >> 2) << 2
+        b = (b >> 3) << 3
+        pixel = (r, g, b)
+
         map = self._pix_map[self._rotation]
         # Two bytes per pixel in fb memory, 16 bit RGB565
         # Our custom module stores the tuples not the 16bit value