Cycles : Interactive displacement edit fix

[boberfly]

Generally describe what this PR will do, and why it is needed

Cycles :

• Fixes a double-up of displacement edits every time the shader was modified.
• Added support for automaticInstancing attribute support (this is an opportunistic addition which is fairly related!)

TODO: I can't seem to trigger the unit-test as the live scene edit isn't actually being applied properly with the update to the sphere object's attributes - is there something I'm missing here?

Related issues

• Partial workaround for Cycles : Shader assignments "leaking" onto identical meshes #4890

Dependencies

• N/A

Breaking changes

• Scene locations with automaticInstancing disabled on scene locations will now not do any instancing, not sure if anyone would be setting this in a Cycles scene prior to this but they might've been and will get a different result.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[boberfly]

Looks like this addresses displacement modifications but not surface shader modifications. Most likely it is this flag that's getting set: https://projects.blender.org/blender/blender/src/commit/4aa18b7242f508c0c81f8d5b5bc480ded955606b/intern/cycles/scene/shader.cpp#L390
So I think we might just need to remove that optimisation and just re-issue a mesh for all shader modifications unless I can come up with a workaround or modify Cycles itself...