Added BC5U reading

[radarhere]

#6486 refactors the DDS plugin, but also adds support for reading BC5U images. This PR adds that new support, without refactoring.

https://walbourn.github.io/the-dds-file-format-lives/ mentions

a FourCC of “BC5U” for the DXGI_FORMAT_BC5_UNORM format

Here are the settings we currently use for that format as part of DX10

Pillow/src/PIL/DdsImagePlugin.py

Lines 189 to 192 in 2460621

	if dxgi_format in (DXGI_FORMAT_BC5_TYPELESS, DXGI_FORMAT_BC5_UNORM):
	self.pixel_format = "BC5"
	n = 5
	self._mode = "RGB"

So those same settings can be used when fourcc is "BC5U", making it the same as our existing fourcc of "ATI2"

Pillow/src/PIL/DdsImagePlugin.py

Lines 176 to 179 in 2460621

	elif fourcc == b"ATI2":
	self.pixel_format = "BC5"
	n = 5
	self._mode = "RGB"

I created a test image by copying and hexeditting our ATI2 test image.