Add public build pipelines for the core implementation and the C++ and C# libraries:
- [`mr-webrtc-core`](https://dev.azure.com/aipmr/MixedReality-WebRTC-CI/_build?definitionId=23) builds the core Google implementation and UWP wrappers (manually triggered when needed).
- [`mr-webrtc-cpp-ci`](https://dev.azure.com/aipmr/MixedReality-WebRTC-CI/_build?definitionId=24) builds the C++ library when a change is pushed to it.
- [`mr-webrtc-cs-ci`](https://dev.azure.com/aipmr/MixedReality-WebRTC-CI/_build?definitionId=25) builds the C# library when a change is pushed to it.
- [`mr-webrtc-docs-ci`](https://dev.azure.com/aipmr/MixedReality-WebRTC-CI/_build?definitionId=23) builds the documentation (user manual and API reference) when a change is pushed to the `docs/**` sub-tree or to any C# file, then commit the changes to the `gh-pages` branch, a special branch that GitHub uses as sources for the online documentation https://microsoft.github.io/MixedReality-WebRTC (currently empty, and committing to the not-so-special `gh-pages-private` instead, until making the repository public).

Azure Hosted VS2017 agents only have 2.7.14.

Add in Unity's Add Component menu an entry for each MixedReality-WebRTC component.

- Enable manual selection of horizontal mirroring of the video feed
  so that the user can see themself in sync like in a real-world mirror.
- Automatically invert the vertical UV coordinates if on a platform
  where needed, based on UNITY_UV_STARTS_AT_TOP.

Add a `LocalVideoSourceEditor` helper to show an error message when the
`LocalVideoSource` has MRC enabled but not XR, as the later will produce
a 2D slate non-exclusive app which cannot use MRC on HoloLens 2.