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_*().

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4125>

cogl/cogl/cogl-boxed-value.h

@@ -61,6 +61,7 @@ typedef struct _CoglBoxedValue
     CoglBoxedValue *_bv = (bv);                 \
     _bv->type = COGL_BOXED_NONE;                \
     _bv->count = 1;                             \
+    _bv->size = 0;                              \
   } G_STMT_END
 
 char *