Calculate Optical Flow with an ESP32-CAM

TODO:

• adjust focal length

To pull the mavlink libraries:

git submodule init
git submodule update

Resources and documentations:

• esp32:
  https://github.com/espressif/esp32-camera

• camera:
  https://github.com/espressif/arduino-esp32/tree/master/libraries/ESP32/examples/Camera/CameraWebServer

• optical flow:
  https://github.com/PX4/Flow/blob/master/src/modules/flow/flow.c
  https://github.com/PX4/OpticalFlow/blob/master/src/px4flow.cpp

• mavlink:
  https://mavlink.io/en/mavgen_c/
  https://github.com/mavlink/c_library_v2
  https://github.com/mavlink/c_library_v2/blob/master/common/mavlink_msg_optical_flow.h
  https://github.com/mavlink/c_library_v1/blob/master/common/mavlink_msg_optical_flow.h#L200
  https://github.com/mavlink/c_library_v1/blob/master/mavlink_helpers.h#L620
  https://github.com/PX4/Flow/blob/master/src/modules/flow/main.c#L572

• i2c:
  https://github.com/PX4/Flow/blob/master/src/include/i2c_frame.h
  https://github.com/eschnou/arduino-px4flow-i2c

• TFmini Plus (std9040):
  http://en.benewake.com/product/detail/5c345cd0e5b3a844c472329b.html
  https://github.com/senegalo/TFMiniPlus