Add ClutterInterval integration

To be able to animate CLUTTER_TYPE_UNITS properties we need to register
the GType and its progress function against the ClutterInterval code.

The two ClutterUnits defining the interval can use different units, the
resulting unit will always be in pixels, so calculating a progress
between 10px and 4cm is valid.

http://bugzilla.openedhand.com/show_bug.cgi?id=1844

clutter/clutter-units.c

@@ -71,6 +71,7 @@
 
 #include "clutter-units.h"
 #include "clutter-private.h"
+#include "clutter-interval.h"
 
 #define DPI_FALLBACK    (96.0)
 
@@ -646,6 +647,31 @@ clutter_units_to_string (const ClutterUnits *units)
   return g_strconcat (buf, " ", unit_name, NULL);
 }
 
+/*
+ * ClutterInterval integration
+ */
+
+static gboolean
+clutter_units_progress (const GValue *a,
+                        const GValue *b,
+                        gdouble       progress,
+                        GValue       *retval)
+{
+  ClutterUnits *a_units = (ClutterUnits *) clutter_value_get_units (a);
+  ClutterUnits *b_units = (ClutterUnits *) clutter_value_get_units (b);
+  ClutterUnits  res;
+  gfloat a_px, b_px, value;
+
+  a_px = clutter_units_to_pixels (a_units);
+  b_px = clutter_units_to_pixels (b_units);
+  value = progress * (b_px - a_px) + a_px;
+
+  clutter_units_from_pixels (&res, value);
+  clutter_value_set_units (retval, &res);
+
+  return TRUE;
+}
+
 /*
  * GValue and GParamSpec integration
  */
@@ -731,6 +757,9 @@ clutter_units_get_type (void)
       g_value_register_transform_func (G_TYPE_STRING, clutter_units_type,
                                        clutter_value_transform_string_units);
 
+      clutter_interval_register_progress_func (clutter_units_type,
+                                               clutter_units_progress);
+
       g_once_init_leave (&clutter_units_type__volatile, clutter_units_type);
     }