[Feature] Motion Vectors for Universal MVP

[he-ax]

Checklist for PR maker

• 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. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet.
• 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.

---

Purpose of this PR

Adding Motion Vector pass to Universal Render Pipeline. This is needed for URP to support features like Motion Blur and TAA. This is a MVP and has some restrictions.
Does not support custom shaders that modify vertex positions.
Only supports R16G16_SFloat for render target.
Only supports per object motion vectors for objects that are moving.
For 3.0 target and above.
Disabled for xr.

---

Testing status

Motion Vector pass is disabled by default and need to be enabled thorough script. Test scene contains a script that does this and then renders Motion Vectors on top of the regular scene.

Added test scene in _Foundation project: 206_Motion_Vectors

Test displays motion vectors target rendered to the game view. It displays both per object and camera motion vectors, but since the camera is not moving it only shows per object motion vectors right now.

Link to yamato for c++ branch testing against Graphics master:
https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/master/.yamato%252F_abv.yml%2523all_project_ci_CUSTOM-REVISION/6733935/job/full-pipeline
Testing this PR against same trunk revision.

---

Comments to reviewers

Technical Design Document: https://docs.google.com/document/d/1P3HstSS7MFpMW9qT5ty8fC8_c15nTIqgIJxEnomNRS8/edit?usp=sharing

Relies on trunk change: https://ono.unity3d.com/unity/unity/pull-request/125001/_/graphics/srp/motion-vectors-fallback-material

[he-ax]

Is there a better place to keep these shaders?

[nigeljw-unity]

I think it is fine for them to be in this folder, since motion vectors are not specific to only post process.

[Verasl]

LGTM

Would be nice to have more effects to validate but we can do any fixes to this once we look at adding Per Object Motion Blur and TAA etc

[nigeljw-unity]

This would be a nice option to have.

[ernestasKupciunas]

Did some smoke testing on mobiles. Looks good. Visually scene was rendered as expected. No errors in the logcat (Android) nor in the Xcode(iOS).
Android Gles3 and Vulkan:

iOS:

Unity used for testing:
Version: 2021.2.0a17.2411.graphics/srp/motion-vectors-fallback-material.10
Revision: graphics/srp/motion-vectors-fallback-material 439ecc0b6d35
Built: Mon, 10 May 2021 12:43:49 GMT

Devices under testing:
iPad Air4, SoC: A14, iOS: 14.0
iPhone 12Pro, SoC: A14, iOS: 14.1
VLNQA00217, Razer Phone 2, 9.0.0, Snapdragon 845 SDM845, Adreno 630
VLNQA00268, Samsung Galaxy S10+, 10.0.0, Exynos 9 9820, Mali-G76
OnePlus Nord, 10, Snapdragon 765G SM7250-AB, Adreno 620

[erikabar]

no issues found. Tested custom project with moving objects, moving camera, camera stacking with renderer feature dev provided in automated scene