Add Virtual texturing support for HDRP [Skip CI]

[GraphineCharles]

** ORIGINAL PR HERE here **

Added original reviewers except:

• Some "Team" reviewers which don't seem to be available (yet?) "Shader Graph" "gfx-aq-bellevue" "Universal RP Team"
• One individual reviewer which doesn't seem available "zadi0648"

DONT FORGET TO ADD A CHANGELOG

Checklist for PR maker

• No backport needed 2020.1 only. (9.x.x preview package)
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR.
• Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test.

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Purpose of this PR

Add virtual texturing support to HDRP.
Unity trunk now has support for VT but without the SRP's driving this it is not possible for user to use this. This PR adds the SRP side of VT for HDRP. This includes two major parts of code

• Feedback analysis: Add an extra rendertarget written during the relevant phases (GBuffer, forward) that allows the VT system to collect information on which textures were accessed by the shaders.
• Shader graph support: VT enabled materials can be created in shadergraph through a new texture stack node. If VT is enabled in the project this will be using VT if VT is disabled this will fall back to using regular 2D texture sampling.

For more information on virtual texturing please see:

• A presentation on Virtual Texturing at the graphics summit (November) here https://drive.google.com/open?id=1wLsvSlUoy9rqcoQ9xEUiRO-tn5n_yzCW
• A draft of the Streaming Virtual Texturing User Guide here https://docs.google.com/document/d/1pzKz7bCPTjAvMcpUlxOs83KXwKZKC7rdTlBqwGIbOfU/edit
• A presentation about the state of Procedural Virtual Texturing here https://drive.google.com/file/d/1EmnYDMAVwMzuoDE8Mg6KKwCVWMJBLBSQ/view
• A sample project demonstrating Procedural VT here https://github.cds.internal.unity3d.com/unity/VTTestProjects/tree/master/PVTExampleProject
• A sample project demonstrating Streaming VT (a bit rough, we use it to develop) here https://github.cds.internal.unity3d.com/unity/VTTestProjects/tree/master/HDRP

Note some scenes in the sample project are broken due to the non VT related bug:
https://fogbugz.unity3d.com/f/cases/1230818/

Note II: documentation is not part of this PR, as this is a bigger feature needing docs both in trunk, and "backstory" documentation we're still working on this with the documentation team.

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Testing status

Manual Tests: What did you do?

• Opened test project + Run graphic tests locally
• Built a player
• Checked new UI names with UX convention
• Tested UI multi-edition + Undo/Redo + Prefab overrides + Alignment in Preset
• C# and shader warnings (supress shader cache to see them)
• Checked new resources path for the reloader (in devloper mode, you have a button at end of resources that check the pathes)
• Other:

Automated Tests: What did you setup? (Add a screenshot or the reference image of the test please)

• Graphics test 9713

Yamato: (Select your branch):
https://yamato.prd.cds.internal.unity3d.com/jobs/78-Graphics/tree/VirtualTexturing

Any test projects to go with this to help reviewers?

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Comments to reviewers

Notes for the reviewers you have assigned.