GPU: Test if we can remove depth texture array restriction

[thatcosmonaut]

Currently we prevent clients from creating 2D depth texture arrays, but I actually suspect that there's no real reason to have this restriction. It might just work with the code we already have, we just need to write an example that uses it and make sure it works as expected on the different backends.

[slouken]

@thatcosmonaut, is this likely to make 3.4? We're hoping to ship sometime this month.

[flibitijibibo]

Since this is mostly a runtime change rather than a full API change/addition we can skip this for 3.4 - it isn't blocking anything in production AFAIK.