Finds the features in this at
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 features inside a constant 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
, you may modify the features returned in the usual way, e.g. with functions like
the index of the structure
a pointer to the CapsFeatures corresponding to