message: OpenGL: implement a synchronous MapReadAsync
  Mapping OpenGL buffers persistently and unsynchronized isn't supported
  on OSX's OpenGL version where we want to run tests.
diff: @@ -37,12 +37,17 @@ namespace opengl {
  glBufferSubData(GL_ARRAY_BUFFER, start * sizeof(uint32_t), count * sizeof(uint32_t), data);
  }
  - void Buffer::MapReadAsyncImpl(uint32_t, uint32_t, uint32_t) {
  - // TODO(cwallez@chromium.org): Implement Map Read for the GL backend
  + void Buffer::MapReadAsyncImpl(uint32_t serial, uint32_t start, uint32_t count) {
  + // TODO(cwallez@chromium.org): this does GPU->CPU synchronization, we could require a high
  + // version of OpenGL that would let us map the buffer unsynchronized.
  + glBindBuffer(GL_ARRAY_BUFFER, buffer);
  + void* data = glMapBufferRange(GL_ARRAY_BUFFER, start, count, GL_MAP_READ_BIT);
  + CallMapReadCallback(serial, NXT_BUFFER_MAP_READ_STATUS_SUCCESS, data);
  }
  void Buffer::UnmapImpl() {
  - // TODO(cwallez@chromium.org): Implement Map Read for the GL backend
  + glBindBuffer(GL_ARRAY_BUFFER, buffer);
  + glUnmapBuffer(GL_ARRAY_BUFFER);
  }
  void Buffer::TransitionUsageImpl(nxt::BufferUsageBit, nxt::BufferUsageBit) {
  