cssvalue: Add _gtk_css_value_transition()

Returns a value that transitions between start and end or %NULL if the
values cannot be transitioned.

So far, all implementations but numbers and rgba return NULL.

gtk/gtkcssenumvalue.c

@@ -42,6 +42,14 @@ gtk_css_value_enum_equal (const GtkCssValue *enum1,
   return enum1 == enum2;
 }
 
+static GtkCssValue *
+gtk_css_value_enum_transition (GtkCssValue *start,
+                               GtkCssValue *end,
+                               double       progress)
+{
+  return NULL;
+}
+
 static void
 gtk_css_value_enum_print (const GtkCssValue *value,
                           GString           *string)
@@ -54,6 +62,7 @@ gtk_css_value_enum_print (const GtkCssValue *value,
 static const GtkCssValueClass GTK_CSS_VALUE_BORDER_STYLE = {
   gtk_css_value_enum_free,
   gtk_css_value_enum_equal,
+  gtk_css_value_enum_transition,
   gtk_css_value_enum_print
 };
 
@@ -107,6 +116,7 @@ _gtk_css_border_style_value_get (const GtkCssValue *value)
 static const GtkCssValueClass GTK_CSS_VALUE_FONT_STYLE = {
   gtk_css_value_enum_free,
   gtk_css_value_enum_equal,
+  gtk_css_value_enum_transition,
   gtk_css_value_enum_print
 };
 
@@ -153,6 +163,7 @@ _gtk_css_font_style_value_get (const GtkCssValue *value)
 static const GtkCssValueClass GTK_CSS_VALUE_FONT_VARIANT = {
   gtk_css_value_enum_free,
   gtk_css_value_enum_equal,
+  gtk_css_value_enum_transition,
   gtk_css_value_enum_print
 };
 
@@ -198,6 +209,7 @@ _gtk_css_font_variant_value_get (const GtkCssValue *value)
 static const GtkCssValueClass GTK_CSS_VALUE_FONT_WEIGHT = {
   gtk_css_value_enum_free,
   gtk_css_value_enum_equal,
+  gtk_css_value_enum_transition,
   gtk_css_value_enum_print
 };
 
@@ -261,6 +273,7 @@ _gtk_css_font_weight_value_get (const GtkCssValue *value)
 static const GtkCssValueClass GTK_CSS_VALUE_AREA = {
   gtk_css_value_enum_free,
   gtk_css_value_enum_equal,
+  gtk_css_value_enum_transition,
   gtk_css_value_enum_print
 };