Allow drawing of 1 BPP images with transparency

[rfuest]

This issue came up on the Matrix channel: It isn't currently easy to draw a 1 BPP image where the image data is used as an on-off alpha channel.

This feature could be implemented by the user, if images are providing a pixel iterator (see #609). But a more flexible solution would be to implement a DrawTarget adapter, which would work with all ImageDrawables and is already used by MonoFont: https://github.com/embedded-graphics/embedded-graphics/blob/master/src/mono_font/draw_target.rs

When a DrawTarget adapter is added it would also be possible to add an ImageDrawable wrapper, which internally uses the new DrawTarget adapter:

let image_1bpp = ImageRaw::<BinaryColor>::new(...);
let transparent_image = TransparentImage::new(image_1bpp, Rgb565::RED);

// transparent_image implements ImageDrawable<Color = Rgb565>
// and can be used like any other image
Image::new(&transparent_image, Point::zero()).draw(&mut display);