Fix Editor Issue for LiftGammeGain and ShadowsMidtonesHighlights

[johnpars]

Purpose of this PR

There was a small issue that popped up with the trackball UI utility, where a material was being created in a static constructor.

Case: https://fogbugz.unity3d.com/f/cases/1392513/

I'm targeting the PR against master (instead of hd/bugfix) since I believe there was a recent change to the volume system that made this static constructor initialization an issue, so a backport should not be necessary.

This PR fixes the issue by moving the material's initialization from static constructor to its first use.

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Testing status

I manually verified that adding these two volume components works again, and double checked that the new material initialization approach works as intended.

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Comments to reviewers

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[github-actions]

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[remi-chapelain]

Double checked that no other post processing override used these trackball. ✔️