EogScrollView: Implement simple two-pass filtering

Show the filtered image only after a short time.
This should improve the UI's responsiveness quite a bit.

https://bugzilla.gnome.org/show_bug.cgi?id=665897

origin commit:
https://gitlab.gnome.org/GNOME/eog/commit/88c4f54
https://gitlab.gnome.org/GNOME/eog/commit/8169e0a

src/eom-scroll-view.c

@@ -147,6 +147,10 @@ struct _EomScrollViewPrivate {
 	GdkRGBA *override_bg_color;
 
 	cairo_surface_t *background_surface;
+
+	/* Two-pass filtering */
+	GSource *hq_redraw_timeout_source;
+	gboolean force_unfiltered;
 };
 
 static void scroll_by (EomScrollView *view, int xofs, int yofs);
@@ -1182,6 +1186,45 @@ eom_scroll_view_focus_out_event (GtkWidget     *widget,
 	return FALSE;
 }
 
+static gboolean _hq_redraw_cb (gpointer user_data)
+{
+	EomScrollViewPrivate *priv = EOM_SCROLL_VIEW (user_data)->priv;
+
+	priv->force_unfiltered = FALSE;
+	gtk_widget_queue_draw (GTK_WIDGET (priv->display));
+
+	priv->hq_redraw_timeout_source = NULL;
+	return G_SOURCE_REMOVE;
+}
+
+static void
+_clear_hq_redraw_timeout (EomScrollView *view)
+{
+	EomScrollViewPrivate *priv = view->priv;
+
+	if (priv->hq_redraw_timeout_source != NULL) {
+		g_source_unref (priv->hq_redraw_timeout_source);
+		g_source_destroy (priv->hq_redraw_timeout_source);
+	}
+
+	priv->hq_redraw_timeout_source = NULL;
+}
+
+static void
+_set_hq_redraw_timeout (EomScrollView *view)
+{
+	GSource *source;
+
+	_clear_hq_redraw_timeout (view);
+
+	source = g_timeout_source_new (200);
+	g_source_set_callback (source, &_hq_redraw_cb, view, NULL);
+
+	g_source_attach (source, NULL);
+
+	view->priv->hq_redraw_timeout_source = source;
+}
+
 static gboolean
 display_draw (GtkWidget *widget, cairo_t *cr, gpointer data)
 {
@@ -1307,13 +1350,29 @@ display_draw (GtkWidget *widget, cairo_t *cr, gpointer data)
 	} else
 #endif /* HAVE_RSVG */
 	{
+		cairo_filter_t interp_type;
+
+		if(!DOUBLE_EQUAL(priv->zoom, 1.0) && priv->force_unfiltered)
+		{
+			interp_type = CAIRO_FILTER_NEAREST;
+			_set_hq_redraw_timeout(view);
+		}
+		else
+		{
+			if (is_zoomed_in (view))
+				interp_type = priv->interp_type_in;
+			else
+				interp_type = priv->interp_type_out;
+
+			_clear_hq_redraw_timeout (view);
+			priv->force_unfiltered = TRUE;
+		}
 		cairo_scale (cr, priv->zoom, priv->zoom);
 		cairo_set_source_surface (cr, priv->surface, xofs/priv->zoom, yofs/priv->zoom);
 		cairo_pattern_set_extend (cairo_get_source (cr), CAIRO_EXTEND_PAD);
-		if (is_zoomed_in (view))
-			cairo_pattern_set_filter (cairo_get_source (cr), priv->interp_type_in);
-		else if (is_zoomed_out (view))
-			cairo_pattern_set_filter (cairo_get_source (cr), priv->interp_type_out);
+		if (is_zoomed_in (view) || is_zoomed_out (view))
+			cairo_pattern_set_filter (cairo_get_source (cr), interp_type);
+
 		cairo_paint (cr);
 	}
 
@@ -1853,6 +1912,8 @@ eom_scroll_view_dispose (GObject *object)
 	view = EOM_SCROLL_VIEW (object);
 	priv = view->priv;
 
+	_clear_hq_redraw_timeout (view);
+
 	if (priv->idle_id != 0) {
 		g_source_remove (priv->idle_id);
 		priv->idle_id = 0;