Increase frame rate

[TomCC7]

Even without the issue discussed in #10, the frame rate is still slow for unknown reason. I have tested that when using the demo program with no sensor registration, the frame rate is about 20fps. The condition is even worse when creating multiple vehicles. The frame rate will be decreased to about 10fps when there's 3 cars. Need to discuss this during the meeting. Some stats:

1. CPU: about 200%
2. GPU: about 30%
3. quality: epic(tested with lowest quality and the frame rate increase to 26...)
   

[TomCC7]

Seems that the bottleneck is at CPU side since the GPU never runs in full speed

[TomCC7]

HARPLab/DReyeVR#10 this link gives hardware recommendations. For our use case what is really important is the single-thread performance, so an Xeon CPU won't be that good.

[TomCC7]

carla-simulator/carla#2617 this link gives comprehensive discussion over the performance issue