Feature add: const from_rgb for all colors

[LuckyTurtleDev]

Currently it is very hard to convert colors at compile time.
If you know the type already you can do this conversion manual .
But if you do not know the type yet you can not simple create a Bgr888 and call .into(), because it is not const.
A solution for this would be to add a const from_rgb(r, g,b) -> Self or const new_rgb(r, g,b) -> Self function to each color type (including the grays). So you can simple enter your "normal" 8bit color and it would be converted at compile time. The function could be implementation by using the already existing, but not public const functions.
This would very useful for usage inside macros. Current macros version can only create GrayColor, BinaryColor or PixelColor, but not for all at the same time, because they have a different new functions and need a special solution for each of them.

[mastercaution]

I'm also missing such a function.
The function you linked with_rgb888 is actually exactly what you want ... but it's private and I wonder why.

Making it public would allow for convenient const conversion like that:

const MATERIAL_GREEN: Rgb565 = Rgb565::with_rgb888(76, 175, 80);