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Add a seprate qtheta offset for two-phase stereo.

9b34bd83 caused serious regressions for 240-sample frame stereo,
 because the previous qb limit was _always_ hit for two-phase
 stereo.
Two-phase stereo really does operate with a different model (for
 example, the single bit allocated to the side should really
 probably be thought of as a sign bit for qtheta, but we don't
 count it as part of qtheta's allocation).
The old code was equivalent to a separate two-phase offset of 12,
 however Greg Maxwell's testing demonstrates that 16 performs
 best.

--- a/libcelt/bands.c

+++ b/libcelt/bands.c

@@ -769,7 +769,7 @@

       /* Decide on the resolution to give to the split parameter theta */

       pulse_cap = m->logN[i]+(LM<<BITRES);

-      offset = (pulse_cap>>1) - (stereo ? QTHETA_OFFSET_STEREO : QTHETA_OFFSET);

+      offset = (pulse_cap>>1) - (stereo&&N==2 ? QTHETA_OFFSET_TWOPHASE : QTHETA_OFFSET);

       qn = compute_qn(N, b, offset, pulse_cap, stereo);

       if (stereo && i>=intensity)

          qn = 1;

--- a/libcelt/rate.c

+++ b/libcelt/rate.c

@@ -212,7 +212,7 @@

                if (C==2)

                {

                   max_bits <<= 1;

-                  offset = (m->logN[j]+(i<<BITRES)>>1)-QTHETA_OFFSET_STEREO;

+                  offset = (m->logN[j]+(i<<BITRES)>>1)-(N==2?QTHETA_OFFSET_TWOPHASE:QTHETA_OFFSET);

                   ndof = 2*N-1-(N==2);

                   /* The average measured cost for theta with the step PDF is

                       0.95164 times qb, approximated here as 487/512. */

--- a/libcelt/rate.h

+++ b/libcelt/rate.h

@@ -43,7 +43,7 @@

 #define BITRES 3

 #define FINE_OFFSET 21

 #define QTHETA_OFFSET 4

-#define QTHETA_OFFSET_STEREO 4

+#define QTHETA_OFFSET_TWOPHASE 16

 #define BITOVERFLOW 30000