Fix setting the mode on a ClutterAlpha created with animation_set_alpha(NULL)

Fix setting the mode on a ClutterAlpha created with animation_set_alpha(NULL)

It previously attempted to set the mode on the alpha using
clutter_animation_set_mode_internal, but this was setting the mode on
priv->alpha. At that point in the code priv->alpha is always NULL.

clutter_animation_set_mode_internal now takes a parameter to specify
which alpha to modify.