Make the data views writable

[jholveck]

Previously (while working on 11.0), we changed the .bgra and .rgb attributes to be read-only memoryviews. We also removed the .raw attribute.

The main reason we made the memoryviews read-only is so that we don't have to worry about cached values of .pixels and .rgb being in-sync with the underlying buffer, if users mutated the pixel data.

However, I now realize that this takes away a possible valuable use case: ctypes. ctypes can only create an array (or pointer) from a buffer if it's writable; it doesn't support a read-only version. Previously, users could make a ctypes pointer using the .raw attribute. The new API doesn't give them that ability, without copying the data (or using ctypes' from_address, which is perilous).

Read-only buffers also trigger a warning from PyTorch, although it's clearly-written and only is printed once.

This PR, as it stands, makes the .bgra and .rgb properties return writable memoryviews. (It also restores .raw, this time as a deprecated alias of bgra, since there's no pressing need to remove the name entirely.)

It also documents that, while these memoryviews are writable, actually modifying the data may cause undefined behavior.

There's alternatives, of course. Instead of what I've got here, ChatGPT instead suggests that we leave .bgra and .rgb read-only (as they were in 10.2, since they were bytes objects), and to add a property like .writable_bgra or something. Its argument is as follows:

The problem with making .bgra writable and saying “mutating is UB
[undefined behavior]” is that Python users will absolutely see
writable buffer and think “cool, in-place image editing.” That’s
not UB in the fun C sense; it’s more like “congratulations, you have
a weird cache-invalidation footgun.” The API shape would be lying a
little.

I'm making this PR for discussion and consensus on the best way to go.

Changes proposed in this PR

• Tests added/updated
• Documentation updated
• Changelog entry added
• ./check.sh passed