message: vertexcodec: Add debug trace
  The trace dumps source and encoded data, along with prediction
  information. It needs to be activated using TRACE define.
diff: #include <stdio.h>
  +// #define TRACE
  +
  // This work is based on:
  // TODO: references
  namespace meshopt
  @@ -17,6 +19,66 @@ static unsigned char* encodeVertexBlock(unsigned char* data, const unsigned char
  {
  assert(vertex_count > 0 && vertex_count <= 256);
  +#ifdef TRACE
  + printf("vertex block; count %d\n", int(vertex_count));
  +
  + {
  + for (size_t k = 0; k < vertex_size; ++k)
  + {
  + printf("%02x ", vertex_data[k]);
  + }
  +
  + printf("| base\n");
  + }
  +
  + for (size_t i = 1; i < vertex_count; ++i)
  + {
  + for (size_t k = 0; k < vertex_size; ++k)
  + {
  + size_t vertex_offset = i * vertex_size + k;
  +
  + unsigned char p = vertex_data[vertex_offset - vertex_size];
  +
  + if (prediction && prediction[i])
  + {
  + unsigned char pa = prediction[i] >> 16;
  + unsigned char pb = prediction[i] >> 8;
  + unsigned char pc = prediction[i] >> 0;
  + assert(pa > 0 && pb > 0 && pc > 0);
  +
  + if (pa <= i && pb <= i && pc <= i)
  + {
  + unsigned char va = vertex_data[vertex_offset - pa * vertex_size];
  + unsigned char vb = vertex_data[vertex_offset - pb * vertex_size];
  + unsigned char vc = vertex_data[vertex_offset - pc * vertex_size];
  +
  + p = va + vb - vc;
  + }
  + }
  +
  + printf("%02x/%02x ", vertex_data[vertex_offset], (unsigned char)(vertex_data[vertex_offset] - p));
  + }
  +
  + printf("| ");
  +
  + if (prediction && prediction[i])
  + {
  + unsigned char pa = prediction[i] >> 16;
  + unsigned char pb = prediction[i] >> 8;
  + unsigned char pc = prediction[i] >> 0;
  + assert(pa > 0 && pb > 0 && pc > 0);
  +
  + printf("pgram %d %d %d", pa, pb, pc);
  + }
  + else
  + {
  + printf("delta");
  + }
  +
  + printf("\n");
  + }
  +#endif
  +
  *data++ = static_cast<unsigned char>(vertex_count - 1);
  for (size_t k = 0; k < vertex_size; ++k)
  