The rotation code in the draw_arrow function was assuming that the arrow
would be drawn pointing upwards but it was pointing to the right
resulting in the rotated arrows pointing in the wrong direction.
The recent refactoring caused a pi/2 rotation to be lost. Rather than
adding that back somehwere (to lose it again in the future), we just
draw the arrow pointing upwards as it is expected to do with a 0 angle.
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.
