dummy video encoder for testing

[Antoine Martin]

This encoder will just produce blank frames and consume no CPU.
Useful for testing, this will not be enabled by default.

[Antoine Martin]

Done in r21457 for x264.

To use it:

• --video-encoders=x264 to ensure no other encoders will be used
• XPRA_X264_BLANK_VIDEO=1 to enable this code

We still use the x264 encoder, but it quickly figures out that there is nothing to compress and returns <1KB compressed data.

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Things that differ from a real software / hardware encoder, and things we could do to improve things:

• hardware encoders have to upload memory to the GPU (small cost)
• hardware encoders then download the compressed bitstream from the GPU (small cost)
• the packets we send to the client would normally be much bigger (CPU cost, bandwidth cost, etc)
• the heuristics will find that we compress very well and very fast, that the client decompresses it very fast too - this will have an impact on the speed / quality / batch delay
• we could do the same thing for vpx / x265 / etc
• we could pause in the fake encoder to simulate compression
• we could artificially create larger packets (ie: adding a dummy attribute in the metadata)

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@max: please pass it on for server load testing to see if this makes enough of a difference to the maximum rated user density of your appliances.