[VFX] Add Graphics Buffer support

[PaulDemeulenaere]

Purpose of this PR

Adding Graphics Bufffer support defined by this technical design document.

Add Graphics Buffer in VFX Graph as a first class citizen. Graphics Buffer would be like any other exposed type (except not serialized). This will allow more flexibility and simplicity for users to pass and interpret their own data. At the moment they have to rely on textures for that.

Global approach ⬇️ (the generated code isn't up to date in this screenshot)

• Add two operators :
  
  ⚠️ TODO : document newly added operator.

• Expose VFXType with a new optionnal attribute : Usage.GraphicsBuffer & moved VFXType to runtime, sample of usage :

public struct Rectangle
{
	public Vector2 size;
	public Vector3 color;
}

[VFXType(VFXTypeAttribute.Usage.GraphicsBuffer)]
struct CustomData
{
	public Rectangle rectangle;
	public Vector3 position;
}

⚠️ TODO : document newly exposed API.

---

Testing status

Adding a new graphic test which is displaying a set of rectangle, see this code : https://github.cds.internal.unity3d.com/unity/vfx-graphics/pull/236/files#diff-6e84b835af122c8437c16eb04557d42aR178

Adding several editor test :

• CreateComponent_And_BindGraphicsBuffer : Check bindings & the value of GetNativeBufferPtr consistency
• CreateComponent_And_BindGraphicsBuffer_And_(Reinit) : Check the binding buffer is kept while calling Reinit
• CreateComponent_And_BindGraphicsBuffer_And_(DisableAndRenable) : Check the binding buffer is kept while disabling/renabling visualEffect
• CreateComponent_And_BindGraphicsBuffer_And_(ChangeVisualEffectAsset) : Switch to another visualEffectAsset (which also have the same buffer exposed) (fixed by this C++ change)
• CreateComponent_And_BindGraphicsBuffer_And_(EditSerializedObject) : Modifying the serialized property (fixed by this C++ change)
• Slot_Sanitize_When_A_Sub_Slot_Is_Missing_Link : Cover a behavior added in VFXSlot.cs, we can correctly sanitize if VFXType change its structure adding a field, if slot count doesn't correspond, try to copy by field name.

Tested locally some invalid VFXType :

• Private field aren't supported in VFXType if GraphicsBuffer usage

public struct Type_Using_Private_And_GraphicsBuffer
{
	public int a;
	private int b;
}

• Use another struct which isn't a VFXType

{
	public int a;
}

[VFXType]
public struct Type_Using_Not_VFX_Type_SubType
{
	public float a;
	public NotAVFXType b;
}

• Unsupported blittable type by the VFX

public struct Type_Using_Not_Supported_Type_By_VFX
{
	public float a;
	public byte b;
}

• Not blittable type

public struct Type_Not_Blittable
{
	public float a;
	public Texture2D b;
}

Note : I'm expecting other error case but I tried to cover the most obvious possible incompatibilities.

I've run yamato to check the expected result :

• VFX_URP private
• VFX_URP public
• VFX_HDRP private
• VFX_HDRP private

⚠️ TODO : upload image references ⚠️

---

Comments to reviewers

⚠️ This PR require C++ code : https://ono.unity3d.com/unity/unity/pull-request/123849/_/graphics/vfx/feature/add-graphics-buffer-support (not final, still an ownership issue to resolve see this conversation)

Known limitation :

• We aren't checking the type of buffer, GraphicsBuffer.Target.Structured is kind of expected view

Known issue (wip) :

• The inspector is displaying the GraphicsBuffer

[VladNeykov]

Looking good, no outstanding issues
test doc

[PaulDemeulenaere]

The needed change in C++ landed (see https://ono.unity3d.com/unity/unity/changeset/5676fbb6fbea) , launching Yamato on really last trunk : https://ono.unity3d.com/unity/unity/changeset/281d883a3da809b5684d799dea25486c92813302 (master didn't update the trunk head yet) on d9458b4 ⏳