app, libgimpwidgets, plug-ins: GimpColorArea is now space-invaded.

This includes improvements on the out-of-gamut colored corner being shown for
unbounded component types out of the [0; 1] range (with some small margin of
error to avoid e.g. a -0.0000001 value to show as out-of-gamut).

There are still improvements to be made on the color rendering. In particular,
it still draws as CAIRO_FORMAT_RGB24 cairo surface. We should probably move to
draw as CAIRO_FORMAT_RGBA128F eventually (more precision and even allowing to
draw unbounded colors with a possible option, instead of always clipping).

Also adding the libgimpwidgets API gimp_widget_get_render_space().

libgimpwidgets/gimpwidgetsutils.c

@@ -1055,6 +1055,47 @@ gimp_widget_get_color_transform (GtkWidget               *widget,
   return NULL;
 }
 
+/**
+ * gimp_widget_get_render_format:
+ * @widget: (nullable): [class@Gtk.Widget] to draw the focus indicator on.
+ * @config: the color management settings.
+ * @softproof: whether the color must also be soft-proofed.
+ *
+ * Gets the Babl format to use as target format when rendering raw data on
+ * @widget. The format model with be "R'G'B'A" and the component types will be
+ * double.
+ *
+ * If @config is set, the color configuration as set by the user will be used,
+ * in particular using any custom monitor profile set in preferences (overriding
+ * system-set profile). If no such custom profile is set, it will use the
+ * profile of the monitor @widget is displayed on and will default to sRGB if
+ * @widget is %NULL.
+ *
+ * Use [func@Gimp.get_color_configuration] to retrieve the user
+ * [class@Gimp.ColorConfig].
+ *
+ * TODO: @softproof is currently unused.
+ *
+ * Since: 3.0
+ **/
+const Babl *
+gimp_widget_get_render_space (GtkWidget       *widget,
+                              GimpColorConfig *config)
+{
+  GimpColorProfile *dest_profile  = NULL;
+  const Babl       *space         = NULL;
+
+  g_return_val_if_fail (widget == NULL || GTK_IS_WIDGET (widget), NULL);
+
+  _gimp_widget_get_profiles (widget, config, NULL, &dest_profile);
+
+  if (dest_profile)
+    space = gimp_color_profile_get_space (dest_profile,
+                                          GIMP_COLOR_RENDERING_INTENT_RELATIVE_COLORIMETRIC,
+                                          NULL);
+  return space;
+}
+
 /**
  * gimp_widget_set_native_handle:
  * @widget: a #GtkWindow