timing of point cloud rendering

Hi!

Thank you for the nice library.

Experimenting with the visualizer's function _custom_draw_geometry_with_camera_trajectory_, I get the following timings:

- capture nothing in the callback, 20ms to render;
- capture rgb, 40 ms;
- capture depth, 60 ms;
- capture depth and rgb, 80 ms.

Why is the time for depth capture so big compared to the time for rgb capture? Can I make it faster?

In general, I'm looking for a way to render point clouds as fast as possible.

Thank you!