Missing implementation for From<Rgb555> and From<Rgb565>

[GioPan04]

Hi,
I'm trying to render a BMP image to a 7-color waveshare display.

The embedded-graphics has support for BMP images using the tinybmp crate, and it allows to specify a color space (eg Bmp::<OctColor>::from_slice(...)) but OctColor needs to implement From<Rgb555> and From<Rgb565>.
Right now this crate implements only From<embedded_graphics_core::pixelcolor::Rgb888>.

The implementation could be similar to the Rgb888 one, that tries to find the closest colors.
I'd make a PR but sadly I'm not familiar with graphics at all.

Just for testing I created an implementation for From<Rgb555> and From<Rgb565> that simply converts the value into Rbg888 and use its From implementation to convert the image.
I tested it with this image from the Waveshare repo example that was already converted to the correct color table, and just worked. Of course it needs a better implementation but it works.

[kolypto]

To draw a bitmap, you'd use something like this:

// Display image
let bmp_data = include_bytes!("../../misc/ferris.bmp");
let bmp = tinybmp::Bmp::from_slice(bmp_data).unwrap();
let image = embedded_graphics::image::Image::new(&bmp, Point::new(25, 60));
image.draw(&mut display).unwrap();

and will immediately suffer from the following errors:

error[E0277]: the trait bound `epd_waveshare::color::Color: From<Rgb555>` is not satisfied
104 |     image.draw(&mut display).unwrap();
    |           ^^^^ the trait `From<Rgb555>` is not implemented for `epd_waveshare::color::Color`
error[E0277]: the trait bound `epd_waveshare::color::Color: From<Rgb565>` is not satisfied
104 |     image.draw(&mut display).unwrap();
    |           ^^^^ the trait `From<Rgb565>` is not implemented for `epd_waveshare::color::Color`

error[E0277]: the trait bound `epd_waveshare::color::Color: From<Rgb555>` is not satisfied
103 |     let image = embedded_graphics::image::Image::new(&bmp, Point::new(25, 60));
    |                 ------------------------------------ ^^^^ the trait `From<Rgb555>` is not implemented for `epd_waveshare::color::Color`
    |                 |
    |                 required by a bound introduced by this call

error[E0277]: the trait bound `epd_waveshare::color::Color: From<Rgb555>` is not satisfied
102 |     let bmp = tinybmp::Bmp::from_slice(bmp_data).unwrap();
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<Rgb555>` is not implemented for `epd_waveshare::color::Color`

Here's how to fix: use the master branch:

# epd-waveshare = { version = "0.6.0", features = ["graphics"] }
epd-waveshare = { git="https://github.com/caemor/epd-waveshare.git", features = ["graphics"]}

because it has this commit that implements the missing traits for colors.