[core] Adjust RNG calls in synthutils to not truncate

[WinterSolstice8]

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What does this pull request do?

Closes #9345
Addresses part of #7841

Steps to test these changes

Synth, still get HQs.

[zach2good]

We use 1.0f float syntax much more commonly than 1.f, otherwise looks very sane 👍

[WinterSolstice8]

We use 1.0f float syntax much more commonly than 1.f, otherwise looks very sane 👍

Changed