fix: Upside down screenshare from Unity VR app

[NiekVRelax]

Problem

When using the Unity LiveKit SDK with a VR app as the sender, the screenshare on the receiving end is flipped upside down.

Solution

Use a RenderTexture with a Bilt approach instead of a Texture2D for gathering the pixels.

Notes

This was tested with version 1.3.3 of the SDK in a VR Unity app.

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