Refactor VFX / SG Integration (use any target)

[johnpars]

Purpose of this PR

Allow users to target VFX Graph with HDRP Master Node/Stack in ShaderGraph. Specifically it unlocks the following to users:

• Support any HDRP material type (StackLit, Hair, etc.) for any VFX Output Context.
• Support Vertex Stack in VFX (Replacing this PR).
• Improve VFX Output Context UI by re-using Material UI blocks for surface option configuration.

Bubbles.mp4

The refactor is done with the following technical goals in mind:

1. Consolidate the various VFX .template files into a single template, reducing code duplication and making VFX more easily maintainable if there are changes to HDRP shading architecture.
2. Phase out the need for HDRP's specific VFX Context implementations (i.e. the "Output Particle Lit Mesh" etc.). Output contexts themselves should only describe the primitive/task type and not also suggest the material type- this is the responsibility of the shader graph.
3. Use Shader Graph as the VFX shader generator and share as much information as possible with the render pipeline's (sub)target. This lets VFX automatically scale with HDRP whenever a new sub-target/material type is introduced. (Prior to this PR, the Visual Effect target would have to manually re-implement the work for each material type- and so far only supported lit/unlit).
4. Serialize the generated VFX material within the VFX asset. Prior to this PR the material was generated on the fly and all of the render state was statically known at compile time. The problem with this is VFX must re-implement all of the render state configuration itself. By serializing the material we can easily re-use the HDRP material inspector UI blocks which are largely responsible for configuring render state and surface options.
5. Keep the original VFX Shader Graph Integration. For now, this PR simply offers a new generation path, with plans for a future PR to support URP as well (for 21.2).

Usage

1. Ensure Improved ShaderGraph Generation is enabled in the Visual Effects Preferences panel. It should be enabled by default.

2. Select the new Support VFX Graph toggle in within the HDRP Target Graph Inspector and save the asset.

3. In VFX Graph, assign the VFX compatible shader graph into the Shader Graph field. Note that exposed fragment and vertex properties in the Shader Graph will be exposed in the Graph View and configurable per-particle (this is exactly how the current SG support works).

4. Configure the effect's render state & surface properties in the output context's inspector. Note that we now serialize materials within the VFX asset, and can re-use the material inspector generated by the shader graph. This allows us to holistically configure render state in HDRP between VFX and normal material renderers, and convey that information to the user in a familiar way.

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Testing

From a user point of view, this PR introduces a slight adjustment in workflow.
From a quality/testing point of view this PR introduces various things that will require validation in various systems (HDRP, SG, VFX).

• Manually verified that all HDRP sub targets compile for all VFX output types (mesh),
• Manually verified motion vectors for all output types.
• Manually verified support for vertex stack (and motion vectors for vertex stack + VFX animation at the same time).
• Manually verified render state configuration via the new Context UI's material inspector.
• Manually verified multiple output contexts in the same system.
• Manually verified the new workflow of toggling VFX support in graph, and linking to it from VFX.
• Manually verified support for the original Visual Target / integration.
• Made a standalone build
• Test Document
• Yamato
• Added new test 34_ShaderGraphGeneration to VFX_HDRP. @VladNeykov plans to add more robust testing in the FTP.

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Comments to reviewers

• Note that this PR does not support the HDRP Decal target. (though should, with some more work).
• I have left various specific remarks in the PR for discussion.
• This PR is dependent on an incoming C++ PR from the VFX team for VFX Material Serialization. In the meantime I have made a temporary workaround C++ branch that injects the material render state: https://ono.unity3d.com/unity/from-git/changelog?branch=graphics%2Fvfx%2Finject-material-state
• The PR is dependent on this C++ PR to land the material serialization: https://ono.unity3d.com/unity/unity/pull-request/122141/_/graphics/vfx/feature/material-serialization
• This is a pretty big PR so please only review relevant areas to your domain if you'd like.
• This branch was made at the same time as Shader Graph Custom Interpolators (Custom Interpolator Feature #3281 ). It's unclear how easy it would be to leverage that feature in this PR. Perhaps in a future PR we could use it.

[VladNeykov]

Everything green on the test doc, thanks for the many iterations and your hard work on this @johnpars !

[PaulDemeulenaere]

The head still have an unexpected "#pragma enable_d3d11_debug_symbols"
No real impact on execution beside build size.

[PaulDemeulenaere]

I double check the discussion and everything has been resolve.
LGTM, we have to be sure that Nightly_VFX_HDRP is 🟢 but also HDRP, we got this failure on last run with OSX :

[06:26:38.461 Information] [260/262] TestFinish
	name: UnityEngine.TestTools.Graphics.Tests.ImageAssertTests.PerPixel_BadB_Throws
	result: PASSED
	duration: 0.010073 seconds
[06:26:45.241 Information] LogEntry
	severity: Warning
	message: 61 TextureIDs were not freed
	stacktrace: 
	line: 1734
	file: ./Runtime/GfxDevice/GfxDevice.cpp

However, I don't think it's related to your change...
Great job @johnpars ! This refactor will unleash a lot of technical constraint we had with VisualEffectGraph integration.
FYI, I started to port your approach to URP (see this branch) but it's still in progress. I will come back to you once I have something working in both pipeline.

[sebastienlagarde]

Re-launched nightly with custom revision that should be green: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/HDRP%252Fsg-vfx-integration-improved/.yamato%252Fall-hdrp.yml%2523Nightly_HDRP_CUSTOM-REVISION/6546711/job/pipeline