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.
