fix: screenshot mirroring

[vaind]

fixes #935

edit: this won't work because UNITY_UV_STARTS_AT_TOP is a shader macro - not available as a preprocessor one...

[github-actions]

	Fails
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## Unreleased

- screenshot mirroring ([#1017](https://github.com/getsentry/sentry-unity/pull/1017))

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