The spinner is a regular builtin image now. There is no need to go
through the shadows code manually anymore as regular items do get
shadows automatically.
This also allows simplifying the actual spinner drawing code so that it
actually works.
Instead of supporting every angle, just support top, right, bottom and
left and round the angle to one of those directions.
Adwaita overrides arrows anyway and doesn't even look at the angle, so
this should not be a problem.
Also, make it have a generic entry point with
gtk_css_image_builtin_draw().
The only feature lost so is the drawing of shadows for spinners, but
that will come back later.
... and make it the default. This takes over the meaning from "none" for
this property in that it draws the fallback builtin image.
"none" now literally means no image will be drawn.
