progresstracker: add GTK_SLOWDOWN environment variable

As we consolidate widgets to use progress tracker, this will allow
us to control the speed of all animations in a centralized place

gtk/gtkmain.c

@@ -636,6 +636,7 @@ do_pre_parse_initialization (int    *argc,
                              char ***argv)
 {
   const gchar *env_string;
+  double slowdown;
   
   if (pre_initialized)
     return;
@@ -673,6 +674,13 @@ do_pre_parse_initialization (int    *argc,
 
       g_string_append (gtk_modules_string, env_string);
     }
+
+  env_string = g_getenv ("GTK_SLOWDOWN");
+  if (env_string)
+    {
+      slowdown = g_ascii_strtod (env_string, NULL);
+      _gtk_set_slowdown (slowdown);
+    }
 }
 
 static void

gtk/gtkprivate.h

@@ -91,6 +91,10 @@ gboolean        _gtk_propagate_captured_event  (GtkWidget       *widget,
                                                 GdkEvent        *event,
                                                 GtkWidget       *topmost);
 
+
+gdouble _gtk_get_slowdown ();
+void    _gtk_set_slowdown (gdouble slowdown_factor);
+
 #ifdef G_OS_WIN32
 void _gtk_load_dll_with_libgtk3_manifest (const char *dllname);
 #endif

gtk/gtkprogresstracker.c

@@ -18,6 +18,7 @@
  */
 
 #include "gtkprogresstrackerprivate.h"
+#include "gtkprivate.h"
 #include "gtkcsseasevalueprivate.h"
 
 #include <math.h>
@@ -30,8 +31,25 @@
  *
  * Progress tracker will handle translating frame clock timestamps to a
  * fractional progress value for interpolating between animation targets.
+ *
+ * Progress tracker will use the GTK_SLOWDOWN environment variable to control
+ * the speed of animations. This can be useful for debugging.
  */
 
+static gdouble gtk_slowdown = 1.0;
+
+void
+_gtk_set_slowdown (gdouble factor)
+{
+  gtk_slowdown = factor;
+}
+
+gdouble
+_gtk_get_slowdown (gdouble factor)
+{
+  return gtk_slowdown;
+}
+
 /**
  * gtk_progress_tracker_init_copy:
  * @source: The source progress tracker
@@ -109,7 +127,7 @@ gtk_progress_tracker_advance_frame (GtkProgressTracker *tracker,
       return;
     }
 
-  delta = (frame_time - tracker->last_frame_time) / (gdouble) tracker->duration;
+  delta = (frame_time - tracker->last_frame_time) / gtk_slowdown / tracker->duration;
   tracker->last_frame_time = frame_time;
   tracker->iteration += delta;
 }