cogl/boxed-value: Set size on initialization

Since 1eba07f6 the size of a boxed value is read when calling _cogl_boxed_value_set_x(). This results in an uninitialized read when setting uniforms using cogl_pipeline_set_uniform_*() since the size was not initialized by _cogl_pipeline_override_uniform() when these functions call _cogl_boxed_value_set_*().