SyncPlayer 1.6.8 - visual crop captures a real frame

Fixed

• Visual crop accepted a blank frame as a success. A screenshot of a surface that has not
  painted yet is a valid black PNG, so the crop window opened on nothing. Every capture
  route is now verified for actual picture content, and an empty result is escalated past:
  mpv's own screenshots (video, software, window), then ffmpeg reading the file directly,
  then a window render that works while the player is covered, then hardware decoding
  disabled for one attempt, and finally showing the window (fullscreen if needed) and
  reading the frame off the screen. Anything the last route changes is restored.
• A minimised or hidden player had no way back: the window lookup only matched visible
  windows, so the last-resort route reported no window exactly when it was needed.
• Two capture fallbacks called a helper that does not exist and silently did nothing.
• The wait for a first decodable frame is adaptive, so a stream that is still buffering
  gets a chance, and the failure message names the routes that were tried.