message: samples: update winpack python samples launcher
  accepts script parameter (allows drag & drop from 'explorer')
  use script dir instead of current dir (can launch samples from 'samples/dnn')
  added 'pause' to show error messages (about missing numpy) instead of instant closing
diff: @echo off
  -if NOT exist %CD%\..\..\..\build (
  +setlocal enableDelayedExpansion
  +
  +set SCRIPTDIR=%~dp0
  +if NOT exist %SCRIPTDIR%\..\..\..\build (
  echo ERROR: OpenCV Winpack installation is required
  pause
  exit
  )
  :: Path to FFMPEG binary files
  -set PATH=%PATH%;%CD%\..\..\..\build\bin\
  +set PATH=%PATH%;%SCRIPTDIR%\..\..\..\build\bin\
  :: Detect Python binary
  python -V
  @@ -43,5 +46,17 @@ if %ERRORLEVEL% EQU 32 (
  )
  )
  -:: Launch demo
  +:: Don't generate unnecessary .pyc cache files
  +set PYTHONDONTWRITEBYTECODE=1
  +
  +if [%1]==[] goto rundemo
  +%PYTHON% %*
  +set result=%errorlevel%
  +IF %result% NEQ 0 (pause)
  +EXIT /B %result%
  +
  +:rundemo
  %PYTHON% demo.py
  +set result=%errorlevel%
  +IF %result% NEQ 0 (pause)
  +EXIT /B %result%
  