cogl: deprecates cogl_check_extension

OpenGL is an implementation detail for Cogl so it's not appropriate to
expose OpenGL extensions through the Cogl API.

Note: Clutter is currently still using this API, because it is still
doing raw GL calls in ClutterGLXTexturePixmap, so this introduces a
couple of (legitimate) build warnings while compiling Clutter.

cogl/cogl-feature-private.c

@@ -84,8 +84,8 @@ _cogl_feature_check (const CoglFeatureData *data,
                                    namespace, namespace_len);
               g_string_append_c (full_extension_name, '_');
               g_string_append (full_extension_name, extension);
-              if (!cogl_check_extension (full_extension_name->str,
-                                         extensions_string))
+              if (!_cogl_check_extension (full_extension_name->str,
+                                          extensions_string))
                 break;
             }