message: indexcodec: Fix integer conversion warnings
  Not sure why these warnings don't show up in AppVeyor - they do appear
  in VS2017 /W4 builds.
diff: @@ -125,7 +125,7 @@ size_t meshopt_encodeIndexBuffer(unsigned char* buffer, size_t buffer_size, cons
  int fec = (fc >= 0 && fc < 14) ? (fc + 1) : (c == next) ? (next++, 0) : 15;
  - *data++ = fe | (fec << 4);
  + *data++ = static_cast<unsigned char>(fe | (fec << 4));
  if (fec == 15)
  encodeVarInt(data, next - c);
  @@ -148,8 +148,8 @@ size_t meshopt_encodeIndexBuffer(unsigned char* buffer, size_t buffer_size, cons
  int feb = (fb >= 0 && fb < 14) ? (fb + 1) : (b == next) ? (next++, 0) : 15;
  int fec = (fc >= 0 && fc < 14) ? (fc + 1) : (c == next) ? (next++, 0) : 15;
  - *data++ = (fea << 4) | 15;
  - *data++ = (feb << 4) | fec;
  + *data++ = static_cast<unsigned char>((fea << 4) | 15);
  + *data++ = static_cast<unsigned char>((feb << 4) | fec);
  if (fea == 15)
  encodeVarInt(data, next - a);
  @@ -188,7 +188,7 @@ size_t meshopt_encodeIndexBufferBound(size_t index_count, size_t vertex_count)
  // compute number of bits required for each index
  unsigned int vertex_bits = 1;
  - while (vertex_bits < 32 && vertex_count > (1u << vertex_bits))
  + while (vertex_bits < 32 && vertex_count > size_t(1) << vertex_bits)
  vertex_bits++;
  // worst-case encoding is 2 header bytes + 3 varint-7 encoded indices
  