PBR thin instance color attribute needs to be pushed

[BlakeOne]

PBRBaseMaterial is missing the code to push the attribute for thin instance color. This PR just copies the same lines from StandardMaterial.

Here's a PG that shows the issue that's fixed by the PR, where the thin instances are black when they should be red.

Forum: https://forum.babylonjs.com/t/vertex-error-and-no-colors-on-thin-instances-after-upgrade/28492/12

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[sebavan]

Thanks a ton @BlakeOne, just noticed @CedricGuillemet had pushed the exact same fix :-) and I just merged it as it was at the top of the list. I am going to close your PR. Sorry to not have seen it earlier...

[sebavan]

for reference #12333