use of `int` for `flamegpu::id_t`

[ptheywood]

In multiple locations, the custom "id" variable which is defined as message.newVariableID("id") is accessed with methods using <int> rather than <flamegpu::id_t>.

This should be a type mismatch (4 byte unsigned vs 4 byte signed)? but doesn't appear to be caught at runtime in device code, atleast not in pyflamegpu.