Provide wheel without FFmpeg (non-LGPL wheel)

Start by reviewing the opencv-python-headless wheel build and the referenced OpenCV FFmpeg README, especially the WITH_FFMPEG=OFF option. Determine how a non-FFmpeg wheel could be produced and identify the platforms and package variants it would affect; done means the requested wheel is buildable and distributable without FFmpeg.

Currently, if a user would like to use opencv-python in an application that they distribute with PyInstaller, LGPL licenses produce difficulty due to the requirement to be able to swap in different versions of the LGPL'd library. (This is summarized in this PyOxidizer issue).

opencv-python-headless is very close to being compatible because it excludes Qt (also LGPL) but still includes FFmpeg (#349). FFmpeg brings with it a pretty extensive list of requirements on the developer: https://www.ffmpeg.org/legal.html

For developers who want to use all of the other great features of OpenCV without FFmpeg, do you think there could be a solution for hosting a wheel in this way? Or perhaps the -headless wheels could be updated to not include FFmpeg?

Relevant OpenCV ffmpeg README about build options: https://github.com/opencv/opencv/tree/master/3rdparty/ffmpeg

• On Linux and other Unix flavors OpenCV uses default or user-built ffmpeg/libav libraries.
  If user builds ffmpeg/libav from source and wants OpenCV to stay BSD library, not GPL/LGPL,
  he/she should use --enabled-shared configure flag and make sure that no GPL components are
  enabled (some notable examples are x264 (H264 encoder) and libac3 (Dolby AC3 audio codec)).
  See https://www.ffmpeg.org/legal.html for details.

  If you want to play very safe and do not want to use FFMPEG at all, regardless of whether it's installed on
  your system or not, configure and build OpenCV using CMake with WITH_FFMPEG=OFF flag. OpenCV will then use
  AVFoundation (OSX), GStreamer (Linux) or other available backends supported by opencv_videoio module.

  There is also our self-contained motion jpeg codec, which you can use without any worries.
  It handles CV_FOURCC('M', 'J', 'P', 'G') streams within an AVI container (".avi").

• On Windows OpenCV uses pre-built ffmpeg binaries, built with proper flags (without GPL components) and
  wrapped with simple, stable OpenCV-compatible API.
  The binaries are opencv_videoio_ffmpeg.dll (version for 32-bit Windows) and
  opencv_videoio_ffmpeg_64.dll (version for 64-bit Windows).

  The pre-built opencv_videoio_ffmpeg*.dll is:

  • LGPL library, not BSD libraries.
  • Loaded at runtime by opencv_videoio module.
    If it succeeds, ffmpeg can be used to decode/encode videos;
    otherwise, other API is used.

  FFMPEG build includes support for H264 encoder based on the OpenH264 library.
  OpenH264 Video Codec provided by Cisco Systems, Inc.
  See https://github.com/cisco/openh264/releases for details and OpenH264 license.
  OpenH264 library should be installed separatelly. Downloaded binary file can be placed into global system path
  (System32 or SysWOW64) or near application binaries (check documentation of "LoadLibrary" Win32 function from MSDN).
  Or you can specify location of binary file via OPENH264_LIBRARY environment variable.

  If LGPL/GPL software can not be supplied with your OpenCV-based product, simply exclude
  opencv_videoio_ffmpeg*.dll from your distribution; OpenCV will stay fully functional except for the ability to
  decode/encode videos using FFMPEG (though, it may still be able to do that using other API,
  such as Video for Windows, Windows Media Foundation or our self-contained motion jpeg codec).

  See license.txt for the FFMPEG copyright notice and the licensing terms.