Sets the keys for each key frame inside this.
If this does not hold any key frame, key_frames.length key frames will be created; if
this already has key frames, key_frames must have at least as many elements as the number
of key frames.
| this | |
| key_frames |
an array of keys between 0.0 and 1.0, one for each key frame |
| n_key_frames |
the number of values |