Enhanced video recording capability

New features:

• [viewer/Viewer] Add viewer 'attach_camera' / 'detach_camera' mechanism and use it for travelling

Patch and bug fixes:

• [viewer/Viewer] Remove misused signal handling
• [viewer/Viewer] Do not enforce multiprocess spawn on linux
• [viewer/Viewer] Fix meshcat camera using capture_frame
• [viewer/Viewer] Fix logger warning filtering
• [viewer/Viewer] Fix relative camera pose wrong computation
• [viewer/play_trajectory] Fix video recording not working with Gepetto-gui
• [viewer/play_trajectory] Enforce video framerate and file format automatically (30fps, mp4)
• [viewer/play_trajectory] Add video compatibility with HTML5 (can be embedded in markdown...etc)
• [viewer/play_trajectory] Enable robots offset during video recording
• [viewer/play_trajectory] Add support of camera travelling for both video recording and "human" mode
• [viewer/play_trajectory] Enable custom relative camera pose during travelling
• [Gym/Robot] Remove 'depth_array' rendering mode since it is not available
• ... Various other minor bug fixes.