Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
The new function, gtk_render_background_get_clip answers the
question: what pixels are affected if I call gtk_render_background ?
The long-term goal is to have APIs that answer this question for
all rendering primitives.