Added reading 8-bit color DDS images

[radarhere]

Resolves #7423

The issue has found a DDS image that has the DDPF_PALETTEINDEXED8 pixel format

Pillow/src/PIL/DdsImagePlugin.py

Lines 46 to 50 in b723e9e

	# Pixel Format
	DDPF_ALPHAPIXELS = 0x1
	DDPF_ALPHA = 0x2
	DDPF_FOURCC = 0x4
	DDPF_PALETTEINDEXED8 = 0x20

This pixel format is not mentioned at https://learn.microsoft.com/en-us/windows/win32/direct3ddds/dds-pixelformat under "dwFlags", but is at https://learn.microsoft.com/en-us/previous-versions/ms783269(v=vs.85). It is an "8-bit color indexed" image.

Looking at the data in the issue, I guessed that the file structure was the header, followed by 1024 palette bytes, followed by the indexes to the palette. That produced a good result.

To create a test image, I took the header from hopper.dds, manually changed the width, height and pixel format flags, and put together code to write out the palette and indexes of transparent.gif.

[hugovk]

Thanks!

Worth adding to release notes or documenting elsewhere?

[radarhere]

Sure, I've added it to the release notes and the file formats documentation.

[hugovk]

Thanks!