adds Rgb444 and RawU12

[catherinejones]

This PR adds Rgb444 and RawU12 to support 12bit RGB on displays like the ST7789.

This fixes 1/2 of #726 (missing BGR444).

[catherinejones]

(Hi! This is my first PR to embedded-graphics so my apologies if I've missed anything!)

[BroderickCarlin]

It appears CI is failing on this PR just as a result of failing to fetch https://img.shields.io/matrix/rust-embedded-graphics:matrix.org. This is most likely an intermittent error and rerunning CI should allow it to pass

[rfuest]

The Rgb444 type looks good, but the RawU12 storage type will cause issues without additional code changes. There is no support for using RawU12 values for ImageRaws or Framebuffers. At the moment I would suggest to use RawU16 as a storage type of Rgb444, which will allow Rgb444 to used for images and FBs, but with 4 wasted bits per pixel. When/if PR #711 gets merged the RawU12 type could be revisited to store 2 Rgb444 pixels per 3 bytes, without any wasted bits. This packing is also used by some display controllers and would be a good long term addition IMO.

[catherinejones]

@rfuest I'll take a look at adding support to ImageRaw and Framebuffer! I've been using these types locally for a ST7789 display and it's been working relatively well thus far.

@BroderickCarlin I've merged the latest changes into my branch which seemed to prompt CI to run again.

Sorry about the late reply, I guess I don't have email notifications enabled 😅