Add drawing routines

[thejpster]

Add routines for this to the vga-framebuffer crate, and to the API.

• [ ] Line drawing
• [ ] Rectangle drawing
• [ ] Filled rectangle drawing
• [ ] Bitmap copy

[thejpster]

Unsure if this API should be on the FrameBuffer or on some Canvas struct which can be plugged into the FrameBuffer (where at the moment we plug in a mutable reference to an array).

[MabezDev]

If you haven't heard of embedded-graphics yet, it might be of use. It recently got support for colour displays.

[thejpster]

I've heard of it, but I don't know how it would tie in with my weird block colour attributes.

[MabezDev]

Provided you pass a struct that implements the PixelColor trait you can pass whatever you need. Then when you implement the Drawing trait, you will get access to all the stuff you need to actually draw that pixel with a colour.

For example in my ssd1351 driver:

#[cfg(feature = "graphics")]
impl<DI> Drawing<PixelColorU16> for GraphicsMode<DI> 
    where
    DI: DisplayInterface,
{
    fn draw<T>(&mut self, item_pixels: T)
    where
        T: Iterator<Item = drawable::Pixel<PixelColorU16>>,
    {
        let (width, height) = self.display.get_size().dimensions();
        for drawable::Pixel(UnsignedCoord(x, y), color) in item_pixels {
            if x <= width.into() && y <= height.into() {
                self.set_pixel(x, y, color.into_inner());
            }
        }
    }
}

[thejpster]

Ah, the problem is each colour has to apply to a block of 64 pixels. If you have already used, say, red and yellow within a 64 pixel block, it's not defined what should happen if you then want to draw a blue pixel.