Support LWRP/URP

[Helco]

I did not find an issue regarding Leightweight Render Pipeline, for the camera scenario at least it did not work for me. I debugged this issue down to two problems:

1. TextureManager uses the OnPreRender/OnPostRender events by Unity which are not supported in LWRP. It seems to work when adding these lines in the Start method:

            RenderPipelineManager.beginCameraRendering += (_, cam) =>
            {
                if (cam == spectatorViewCamera)
                    OnPreRender();
            };
            RenderPipelineManager.endCameraRendering += (_, cam) =>
            {
                if (cam == spectatorViewCamera)
                    OnPostRender();
            };

2. After that the output is mostly black because the alpha channel in the renderTexture is ignored in the HoloAlpha shader. I fixed it by modifying the blending to:

fixed4 composite = backCol * (1 - _Alpha * frontCol.a) + frontCol * _Alpha * frontCol.a;

[chrisfromwork]

With these two changes were you able to get yourself unblocked? Thank you for piecing together this in depth overview of the changes that are needed.

[Helco]

Yes with these changes I could use the SpectatorView in a LWRP project. I did not test LWRP to its full extent though.

[Helco]

Quick update as #327 and its beautiful! occlusion support broke my LWRP fix, these are the additional steps to take:

• Break up Blur.shader into two and Graphics.Blit commands per blur pass, as GrabPass isn't supported in LWRP
• In OcclusionMask.shader use _CameraDepthTexture instead of _LastCameraDepthTexture
• Enable DepthTexture in the LWRP asset
• Add the LWRP Additional Camera Data component to the camera in the Compositor prefab