This is in preparation for iusing GtkCssStyle instead of GtkStyleContext
in render functions.

This adds two functions for checking whether an axis-aligned
rectangle is completely outside or inside of a rounded box.
These are not trying to be exact, but fast.

GtkThemingEngine just always calls
  gtk_do_render_foo(engine->priv->context, ...)
now. Other than that, the code is unchanged.

Instead, make the functions use a GtkStyleContext argument.

Instead of the old non-CSS implementation. This allows us to get rid of
the very last piece of custom theming in Adwaita.

This is part of a bunch of fixes for gcc complaining about
-Wmissing-declarations.

It puts functions into headers and includes those headers both where the
functions are defined and where they function are used.

... and convert those properties to this value.

And make the base function just use the raw corner radii struct.

We can do % now, wohoo!

Same order as CSS again.

The nice thing about this is that now edge i in a rounded box follows
corner i and is followed by corner (i + 1) % 4.

GtkCssBorderRadius is going away.

Also use this support to implement inset and outset border styles
without cheats.

CSS drawing operates on rounded boxes a lot, so it makes sense to split
them out instead of duplicating them everywhere.