cogl: Introduce private feature flags and check for ARB_fp

The Cogl context has now a feature_flags_private enum that will allow us
to query and use OpenGL features without exposing them in the public
API.

The ARB_fragment_program extension is the first user of those flags.
Looking for this extension only happens in the gl driver as the gles
drivers will not expose them.

One can use _cogl_features_available_private() to check for the
availability of such private features.

While at it, reindent cogl-internal.h as described in CODING_STYLE.

cogl/cogl-feature-private.h

@@ -59,6 +59,9 @@ struct _CoglFeatureData
   const char *extension_names;
   /* A set of feature flags to enable if the extension is available */
   CoglFeatureFlags feature_flags;
+  /* A set of private feature flags to enable if the extension is available
+   * and for internal use only */
+  CoglFeatureFlagsPrivate feature_flags_private;
   /* A list of functions required for this feature. Terminated with a
      NULL name */
   const CoglFeatureFunction *functions;