Eliminate the loop when decoding the split angle.

Use a closed-form formula for the search instead.
This requires an integer sqrt, so it is not actually closed-form,
 but the number of iterations is O(qb) instead of O(2**qb).

libcelt/mathops.c

@@ -41,6 +41,33 @@
 
 #include "mathops.h"
 
+/*Compute floor(sqrt(_val)) with exact arithmetic.
+  This has been tested on all possible 32-bit inputs.*/
+unsigned isqrt32(celt_uint32 _val){
+  unsigned b;
+  unsigned g;
+  int      bshift;
+  /*Uses the second method from
+     http://www.azillionmonkeys.com/qed/sqroot.html
+    The main idea is to search for the largest binary digit b such that
+     (g+b)*(g+b) <= _val, and add it to the solution g.*/
+  g=0;
+  bshift=EC_ILOG(_val)-1>>1;
+  b=1U<<bshift;
+  do{
+    celt_uint32 t;
+    t=((celt_uint32)g<<1)+b<<bshift;
+    if(t<=_val){
+      g+=b;
+      _val-=t;
+    }
+    b>>=1;
+    bshift--;
+  }
+  while(bshift>=0);
+  return g;
+}
+
 #ifdef FIXED_POINT
 
 celt_word32 frac_div32(celt_word32 a, celt_word32 b)