[Feature] Option to define a null TextureID

[jekyllgrim]

GZDoom version

GZDoom 4.12.2

Which game are you running with GZDoom?

None

What Operating System are you using?

None

If Other OS, please describe

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Relevant hardware info

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Is your feature request related to a problem? Please describe.

It's currently not possible to define a null TextureID. In a code block this is solved by manually calling SetNull() or SetInvalid() on a previously declared TextureID, but this doesn't cut it for functions: it's not possible to define an optional TextureID-type argument, since neither TextureID tex = 0 nor TextureID tex = null are recognized.

Describe the solution you'd like

No response

Describe alternatives you've considered

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Add any other context or screenshots about the feature request here.

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