Use proper command buffer synchronization for planet rendering

[nreagan]

this is expensive

// get command buffer for planet rendering (use between begin_frame and begin_main_pass)
plCommandBuffer *cmd_buf = _ext_starter->get_temporary_command_buffer();

// prepare and render planet to offscreen texture
_ext_planet->prepare(planet, cmd_buf);
_ext_planet->render(planet, &camera, cmd_buf);

// submit and CPU-wait — guarantees staging buffer and texture are fully done
_ext_starter->submit_temporary_command_buffer(cmd_buf);