wf-background: Add an option to configure fade duration.

Among other things, this allows disabling fading, by setting the
fade duration to zero.

Author: lcolitti

metadata/background.xml.in

@@ -13,6 +13,10 @@
 		<_short>Cycle Timeout</_short>
 		<default>150</default>
 	</option>
+	<option name="fade_duration" type="int">
+		<_short>Fade Duration</_short>
+		<default>1000</default>
+	</option>
 	<option name="randomize" type="bool">
 		<_short>Randomize</_short>
 		<default>false</default>

src/background/background.cpp

@@ -35,6 +35,12 @@ void BackgroundDrawingArea::show_image(Glib::RefPtr<Gdk::Pixbuf> image,
 
     to_image.x = offset_x / this->get_scale_factor();
     to_image.y = offset_y / this->get_scale_factor();
+
+    fade = {
+        fade_duration,
+        wf::animation::smoothing::linear
+    };
+
     fade.animate(from_image.source ? 0.0 : 1.0, 1.0);
 
     Glib::signal_idle().connect_once([=] ()

src/background/background.hpp

@@ -18,10 +18,9 @@ class BackgroundImage
 
 class BackgroundDrawingArea : public Gtk::DrawingArea
 {
-    wf::animation::simple_animation_t fade{
-        wf::create_option(1000),
-        wf::animation::smoothing::linear
-    };
+    wf::animation::simple_animation_t fade;
+    WfOption<int> fade_duration{"background/fade_duration"};
+
     /* These two pixbufs are used for fading one background
      * image to the next when changing backgrounds or when
      * automatically cycling through a directory of images.