Previously we used zero-cost x264 `X264_CSP_VFLIP` and vpx
`vpx_img_flip` for OpenGL cores (which render their output buffers
upside down). Now the flip is handled client-side during GPU compositing
of the video stream.
This will be needed later, when we have to use more codecs and
Gstreamer.
The main benefit of libyuv, apart from shortening the video pipeline, is quite noticeable latency and CPU usage decrease due to various assembler/SIMD optimizations of the library. However, there is a drawback for macOS systems: libyuv cannot be downloaded as a compiled library and can only be built from the source, which means we should include a cropped source code of the library (~10K LoC) into the app or rise the complexity of macOS dev and run toolchains. The main target system -- Linux, and Windows will use compiled lib from the package managers and macOS will use the lib included as a shortened source-code.
Building the app with the no_libyuv tag will force it to use libyuv from the provided source files.
