Defer initialization of FrameInvariants FrameMEStats

[shssoichiro]

The FrameMEStats arrays are very expensive to create.
We do not always need them in FrameInvariants,
so defering can speed up certain areas of the code
such as scenecut detection.

This reduces the creation time for a FrameInvariants
object by 95%. Scenecut detection time for the standard
method is reduced by 15%.

[shssoichiro]

As it turns out, this also benefits the FrameInvariants created during the encode pass as well. The lookahead_me_stats field is never written to or read from until it is overwritten on https://github.com/xiph/rav1e/blob/master/src/api/internal.rs#L667. So the expensive initialization in FrameInvariants::new is never used.

Hyperfine showed a 2% total speedup for the encode at --speed 9.