This prevents rate reduction due to channel correlation from increasing
by more than 0.25 bit/sample between two frames. This issue was observed
on r.flac where the frame following a transient would lose nearly
1 bit/sample

This only increases the average rate by about 1.5 kb/s, but it
restores the high quality of transients from 1.0. Also it
ensures that dynalloc can never starve other bands when used
aggressively.

1) Stereo saving was being too aggressive because it only considered the LF
   and because the savings were multiplied by coded_bins rather than by
   just the number of "side bins" below the intensity threshold
2) In the case of bandlimited signals, dynalloc would allocate way too many
   bits to the last non-zero band. We now explicitly check for the last
   band with a meaningful signal.

These issues were really obvious when encoding the decoded test01.mp3
because of the strong inter-channel correlation and (especially) the 16 kHz
lowpass used by the mp3 encoder.

This is used to avoid dynalloc doing silly things on periodic LSB
patterns and to reduce the bitrate on near-silence.

tf_estimate was being initialized to zero and then never set because
transient_analysis() wasn't being called. This could also have affected
other frame sizes at lower complexity.

atan2() is replaced by Monty's approximation.

This was found to cause a quality regression (e.g. in the velvet sample)

Tuned the tonality estimator to trigger on signals where only part of the
spectrum is tonal. Also tuned the pitch detector not to be confused
by short-term correlation.

- New DC rejection filter that avoids numerical issues
- Higher cap on HF dynalloc
- Proper filter memory for transient detector
- Prevents transient detector from detecting low-freq tones as transients

Fixes two leakage problems on the wood blocks sample
- Removes DC which causes leakage with no masking
- Detect leakage by comparing short-MDCT energy to long-MDCT energy
  and boost allocation for bands with leakage

Haven't tested it much and not sure not to use it without risk
for high-quality streams that may change bandwidth

Also, reducing the VBR rate on panned mono

Should be mostly usable now.

Conflicts:

	src/opus_encoder.c