attribute: Only unref buffer for buffered attributes
attribute: Only unref buffer for buffered attributes

constant attributes don't have a corresponding buffer so
_cogl_attribute_free shouldn't try to unref it. Also, for good measure,
in the case of constant attributes we should call
_cogl_boxed_value_destroy() (although currently we know there is no
dynamic data associated with the boxed values).

(cherry picked from commit 89d6dc90d10c59676e0deed87c2c15a0c9712737)

Distrotech/cogl,Distrotech/cogl,Distrotech/cogl,Distrotech/cogl