Finds the features in caps that has the index index, and returns it.
WARNING: This function takes a const GstCaps *, but returns a non-const GstCapsFeatures *. This is for programming convenience -- the caller should be aware that structures inside a constant gstreamer.Caps should not be modified. However, if you know the caps are writable, either because you have just copied them or made them writable with Caps.makeWritable, you may modify the features returned in the usual way, e.g. with functions like Caps.featuresAdd.
You do not need to free or unref the structure returned, it belongs to the gstreamer.Caps