attr/m51481: fix insufficient precision in lifting quantization

This commit extends the operating range of the forward and inverse
quantization tables in order to reduce distortion at low bit rates.

tmc3/quantization.h

@@ -78,8 +78,8 @@ inline int64_t
 Quantizer::quantize(int64_t x) const
 {
   // Forward quantisation avoids division by using the multiplicative inverse
-  // with 14 fractional bits.
-  int64_t fracBits = 14 + kFixedPointAttributeShift;
+  // with 18 fractional bits.
+  int64_t fracBits = 18 + kFixedPointAttributeShift;
 
   // NB, the folowing offsets quantizes with a different deadzone to the
   // reconstruction function.

tmc3/tables.cpp

@@ -444,7 +444,7 @@ const uint32_t pcc::kMortonCode256X[256] = {
 
 const int16_t pcc::kQpStep[6] = {161, 181, 203, 228, 256, 287};
 
-const int32_t pcc::kQpStepRecip[6] = {26052, 23173, 20662,
-                                      18396, 16384, 14614};
+const int32_t pcc::kQpStepRecip[6] = {416825, 370767, 330586,
+                                      294337, 262144, 233829};
 
 //============================================================================