Add a function to get affected area for background

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.

gtk/gtkrender.c

@@ -282,6 +282,39 @@ gtk_render_background (GtkStyleContext *context,
   cairo_restore (cr);
 }
 
+/**
+ * gtk_render_background_get_clip:
+ * @context: a #GtkStyleContext
+ * @x: X origin of the rectangle
+ * @y: Y origin of the rectangle
+ * @width: rectangle width
+ * @height: rectangle height
+ * @out_clip: (out): return location for the clip
+ *
+ * Returns the area that will be affected (i.e. drawn to) when
+ * calling gtk_render_background() for the given @context and
+ * rectangle.
+ *
+ * Since: 3.20
+ */
+void
+gtk_render_background_get_clip (GtkStyleContext *context,
+                                gdouble          x,
+                                gdouble          y,
+                                gdouble          width,
+                                gdouble          height,
+                                GdkRectangle    *out_clip)
+{
+  GtkBorder shadow;
+
+  _gtk_css_shadows_value_get_extents (_gtk_style_context_peek_property (context, GTK_CSS_PROPERTY_BOX_SHADOW), &shadow);
+
+  out_clip->x = floor (x) - shadow.left;
+  out_clip->y = floor (y) - shadow.top;
+  out_clip->width = ceil (width) + shadow.left + shadow.right;
+  out_clip->height = ceil (height) + shadow.top + shadow.bottom;
+}
+
 /**
  * gtk_render_frame:
  * @context: a #GtkStyleContext