Fix gpu-particles examples with new enums

[florianvazelle]

Hi @revmischa, just to respond to your comment #199 (comment)

There was a conflict between two of my PRs 😅

I fix the example in this PR, using the new enumeration added in #200

[florianvazelle]

To be clear, as in #158, #200 has introduced breaking changes to the relevant enums. I switch names to uppercase, because they become constants.

To avoid this kind of situation in the future, maybe we should test the build of examples in pull requests?

[revmischa]

yeah there's plenty of room for improvement in the CI!