cogl: Mark helper as static

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4100>

cogl/cogl/cogl-clip-stack.c

@@ -109,7 +109,7 @@ _cogl_clip_stack_entry_set_bounds (CoglClipStack *entry,
 
 /* Transform a homogeneous vertex position from model space to Cogl
  * window coordinates (with 0,0 being top left) */
-void
+static void
 _cogl_transform_point (const graphene_matrix_t *matrix_mv,
                        const graphene_matrix_t *matrix_p,
                        const float             *viewport,

cogl/cogl/cogl-private.h

@@ -73,13 +73,6 @@ typedef enum
   COGL_N_PRIVATE_FEATURES
 } CoglPrivateFeature;
 
-void
-_cogl_transform_point (const graphene_matrix_t *matrix_mv,
-                       const graphene_matrix_t *matrix_p,
-                       const float             *viewport,
-                       float                   *x,
-                       float                   *y);
-
 gboolean
 _cogl_check_extension (const char *name, char * const *ext);