Add conversion from Rgb666 and Bgr666 to Grey8

[brianmay]

This is required for the tinytga library.

Fixes #654.

[jamwaffles]

LGTM but I'll let @rfuest merge in case I missed anything with the colour conversion code.

[rfuest]

The change itself looks good, but there are other instances in the same file where Rgb666 and Bgr666 are also missing. At least the gray_to_rgb and rgb_to_gray should be fixed before this gets merged.

The impl_from_binary! and impl_rgb_to_binary! and their test are also missing the Rgb666 and Bgr666 types. But if you don't want to fix these now, because they are outside the scope of this PR, we can fix these later.

[brianmay]

I am totally unfamiliar with this code, so not surprising if I missed something.

Looks like I missed updating the tests. Tests are good. Now fixed. Hopefully.

[brianmay]

For some reason the CI is unahppy with my new references to Bgr666. Don't understand why, all the tests pass on my box.

This is something I might need help with.

[rfuest]

For some reason the CI is unahppy with my new references to Bgr666. Don't understand why, all the tests pass on my box.

Maybe you forgot to commit something? Rgb666 and Bgr666 are missing in the invocations of the impl_from_binary! (line 121andimpl_rgb_to_binary!` (line 147) macros.

[brianmay]

I missed those places. Now fixed. Odd this didn't break the tests when I ran them locally.

[brianmay]

Tests pass now.

[rfuest]

Odd this didn't break the tests when I ran them locally.

Did you run the tests by calling cargo test? Because that only runs the tests in the main embedded-graphics crate and not the embedded-graphics-core crate. You need to run cargo test --workspace or preferably use just build (if you aren't using Windows) to run all tests.

[rfuest]

Two remaining issues:

• Rgb666 and Bgr666 are missing in the rgb_to_rgb test (line 220)
• Please add a changelog entry to /CHANGELOG.md and /core/CHANGELOG.md

[rfuest]

Looks good, thanks!