Unbreak encoding without producing the reconstructed video.

* Add code to reorder frames during encoding

* Use BTreeMap instead of VecDequeue to hold input frames

* Implement sign bias in MV prediction

* Disable reuse of learned prob and change ref frame semantics

Use LAST2 instead of ALTREF when the second reference frame is also
in the past

- Move the frame information in a separate struct
- Move the EncoderConfig in api.rs

Manually format parts that skip formatting but don't fix the excess length.

Implement ExactSizeIterator and FusedIterator.

70%-90% faster than before according to criterion.

And use it to compute SAD.

* Format SUBSIZE_LOOKUP slightly then skip it

* Use stable but more verbose way to skip rustfmt

* Fix inter for 12-bit input.

12-bit is a special case that rounds differently.

See 7.11.3.2. Rounding variable derivation process in the spec.

Performs a near-exhaustive-equivalent search of sse-optimum deblocking
level setup at speed <= 3.  Currently always uses SSE, psychovisual
(cdef-dist) comes next.

Above speed 3, levels are set by quantizer.

Rather than converting a level to a threshold decision value and testing values against the threshold,
convert values to levels and compare directly against the filter configuration level.
This will allow binning decisions by level in a radix sort.

The belief that a deblocking pass at level zero is a no-op is incorrect;
disable the deblocking of an edge when its filter level is zero

This reverts commit ef97e0ec.

* Call av1_idct4() from inside av1_idct8().

* Call av1_idct8() from inside av1_idct16().

* Use 2 multiply rotation by PI/4.

Adds the full_btr_sqrt2() function that computes a rotation by PI/4
 using 2 multiplies instead of 4.

* Last two frames are used as references and are labeled as LAST
and ALTREF.

* Add rules to include MV with different reference index in MV stack

* Make 2nd reference frame more distant from current frame

* Use correct reference frame in chroma motion compensation

* Enable multiple reference frames only at slower speed settings

* Add unequal quantizer assignment to temporally predicted frames

Allow Github to detect the license

An exhaustive check indicates that over the given parameter range, one of the clamp operations in the narrow deblocking filters is unneded.

Realized on Friday that an exhaustive search can be done in a single
pass by reducing the search to a radix sort; this is yet more
rearrangement of code flow to enable that loop setup.

Also, get a few more-Rusty idioms in place...

Split up more of the setup functions for a deblocking pass so that the
RDO need not perform redundant steps that won't change from test to
test.