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-internal.h

@@ -174,4 +174,12 @@ _cogl_xlib_handle_event (XEvent *xevent);
 
 #endif /* COGL_HAS_XLIB_SUPPORT */
 
+typedef enum _CoglFeatureFlagsPrivate
+{
+  COGL_FEATURE_PRIVATE_ARB_FP = (1 << 0)
+} CoglFeatureFlagsPrivate;
+
+gboolean
+_cogl_features_available_private (CoglFeatureFlagsPrivate features);
+
 #endif /* __COGL_INTERNAL_H */