Support arbitrary masks for uncompressed RGB DDS images

[radarhere]

Resolves #7517

When DDS images contain uncompressed RGB data, they have red, green, blue (and possibly alpha) masks to filter the bits of each pixel into the final value for the channel.

Currently, Pillow only supports masks giving 8 bits per channel.

Pillow/src/PIL/DdsImagePlugin.py

Lines 150 to 157 in e1291b8

	# Texture contains uncompressed RGB data
	masks = {mask: ["R", "G", "B", "A"][i] for i, mask in enumerate(masks)}
	rawmode = ""
	if pfflags & DDPF_ALPHAPIXELS:
	rawmode += masks[0xFF000000]
	else:
	self._mode = "RGB"
	rawmode += masks[0xFF0000] + masks[0xFF00] + masks[0xFF]

The linked issue has found an image with masks that use 5 bits per channel. So this PR adds support for arbitrary masks by adding a dedicated Python decoder.

[akx]

Might be worth it to precalculate the total here to avoid that extra multiplication every loop (since we know the value can't possibly change):

Suggested change

	while len(data) < self.state.xsize * self.state.ysize * len(masks):
	total_size = self.state.xsize * self.state.ysize * len(masks)
	while len(data) < total_size:

[radarhere]

I personally think the current version is simpler to read, and expect the performance difference to be negligible.

The current version is also used in a few other places in Pillow - but at the same time, that's probably because I wrote them.

Pillow/src/PIL/QoiImagePlugin.py

Line 55 in 316f397

while len(data) < self.state.xsize * self.state.ysize * bands:

[akx]

int.from_bytes?

[radarhere]

Thanks, I've pushed a commit.