Implement the partial rendering with winit and our software renderer.
When the background is not opaque, we must still initialize all the
pixel with 0 otherwise we blend over the previous frame.
(That wasn't visible before because the buffer was always empty)
Implement rgb::Pod/rgb::Zeroable for PremultipliedRgbaColor and Rgb565
Pixel. This allows for bytemuck::cast_slice(), making it easier to copy
software renderer output into another buffer that's the same format but
Vec<u8> for example.
Despite the documentation of softbuffer claiming that the high bits
should be 0, it seems to be interpreted as an alpha value when the
window is actually transparent.
And the values are considered as premultiplied alpha
Also use a Premultiplied aplha for the buffer so we can even have
transparent window with the software renderer
Their choice of i16 is specific to the software renderer. In say the Skia renderer
the physical coordinate space would still use Skia's Scalar (floating points).
