#76 board detect usability

Close #76 

Improvements:
- autodetect falls back to manual selection if it completely fails to find a board
- autodetect only modifies the corner points if the user accepts the detection (Not pressing ESC, prevents problems when autodetect finds bogus points)
- refactored rendering text on opencv image
  - it now renders a transparent black box under the text for better readability
  - and the font size is a bit bigger
  - and it can handle new line characters properly now :)