background-content: Add support for rounded clipping when drawing

We're going to round the workspace backgrounds in the new overview for
gnome-shell 40.

So far corner-rounding was only possible for StWidgets because the
rounded clipping was done using cairo drawing. We now need rounded
clipping for ClutterActors too because backgrounds are drawn using
ClutterActors (or more specifically a ClutterContent). To implement
that, first a ClutterOffscreenEffect subclass together with a fragment
shader from GSK (see gskSetOutputColor() [1] in the GSK GL renderer
code) was investigated, and while that was generic and worked quite
well, it was extremely slow for the case of drawing wallpapers because
of all the FBOs that had to be allocated.

This is the new, more performant approach: Use the same fragment shader,
but perform the rounded clipping right in MetaBackgroundContent while
we're painting the wallpaper. This has almost no performance impact,
with the downside of not being a generic solution.

To allow for rounded clipping not only at the edges of the wallpaper,
but using any given bounding rectangle, the API exposes not only the
radius, but also a bounding rect.

[1] https://gitlab.gnome.org/GNOME/gtk/-/blob/master/gsk/resources/glsl/preamble.fs.glsl

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1717>

src/meta/meta-background-content.h

@@ -63,4 +63,12 @@ void meta_background_content_set_vignette (MetaBackgroundContent *self,
                                            double                 brightness,
                                            double                 sharpness);
 
+META_EXPORT
+void meta_background_content_set_rounded_clip_radius (MetaBackgroundContent *self,
+                                                      float                  radius);
+
+META_EXPORT
+void meta_background_content_set_rounded_clip_bounds (MetaBackgroundContent *self,
+                                                      const graphene_rect_t *bounds);
+
 #endif /* META_BACKGROUND_CONTENT_H */