Demo video-capture width and height must be divisible by 2

[sjoerdsmink]

When the resolution is not an even number, it generates an error:

 File "av/codec/context.pyx", line 200, in av.codec.context.CodecContext.open                                                                                                                           
    File "av/codec/context.pyx", line 220, in av.codec.context.CodecContext.open                                                                                                                           
    File "av/error.pyx", line 424, in av.error.err_check                                                                                                                                                   
  av.error.ExternalError: [Errno 542398533] Generic error in an external library: 'avcodec_open2(libx264)'; last error log: [libx264] height not divisible by 2 (1512x949)

Changes proposed in this PR

Fixes #
Width and height must be divisible by 2, otherwise the encoder will fail. Round down to the nearest even number.

• Tests added/updated
• Documentation updated
• Changelog entry added
• ./check.sh passed

[BoboTiG]

/cc @jholveck

[jholveck]

I would suggest changing the capture resolution too; not sure what the libraries will do if they don't match, but making them match is probably best.

In the non-simple demo, I would also conditionalize this on the codec being libx264: not all codecs have this limitation (I just tested on h264_nvenc).