fix serializing mlt_color

Transparent (0, 0, 0, 0) was serializing as "#0", which would then
deserialize to opaque black because it did not have enough chars to
include an alpha component.

src/framework/mlt_property.c

@@ -702,7 +702,7 @@ char *mlt_property_get_string_tf(mlt_property self, mlt_time_format time_format)
             self->prop_string = malloc(10);
             uint32_t int_value = ((self->prop_int & 0xff) << 24)
                                  | ((self->prop_int >> 8) & 0xffffff);
-            sprintf(self->prop_string, "#%x", int_value);
+            sprintf(self->prop_string, "#%08x", int_value);
         } else if (self->types & mlt_prop_double) {
             self->types |= mlt_prop_string;
             self->prop_string = malloc(32);
@@ -802,7 +802,7 @@ char *mlt_property_get_string_l_tf(mlt_property self,
             self->prop_string = malloc(10);
             uint32_t int_value = ((self->prop_int & 0xff) << 24)
                                  | ((self->prop_int >> 8) & 0xffffff);
-            sprintf(self->prop_string, "#%x", int_value);
+            sprintf(self->prop_string, "#%08x", int_value);
         } else if (self->types & mlt_prop_double) {
             self->types |= mlt_prop_string;
             self->prop_string = malloc(32);