rusticl: fix UB in CLProp machinery

rusticl: fix UB in CLProp machinery

Viewing structs as a collection of u8 is not generally sound. Any padding bytes might be
uninitialized and creating an integer from uninitialized memory constitutes producing an invalid
value, which is instant UB.

Since we only copy these bytes around, the fix is to simply work with MaybeUninit<u8>, which can handle uninitialized memory just fine, instead.

See: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23652>