What does this PR do?

This PR adds a new image operation toPIL.ImageOps that applies a
primary-color dithering filter (RGB only).

The filter:

• Thresholds RGB channels to primary colors (0 or 255)
• Applies a simple dithering strategy to preserve local contrast
• Returns a new image, leaving the input unchanged

Why is this useful?

Primary-color dithering can be useful for:

• Educational purposes (image processing fundamentals)
• Artistic and stylized image transformations
• Low-color or hardware-constrained displays

What is included?

• New function:ImageOps.dither_primary
• Unit tests covering basic behavior and output properties
• Documentation added to the ImageOps module

Notes

• The function expects an RGB image
• The implementation follows existing ImageOps conventions

Before Filter

After Filter