Changes in this patch:
- Change cubic texture gradient and hessian functions to also return the texture values [[1d50efe]](https://github.com/mitsuba-renderer/drjit/commit/1d50efecaad7afac71e32ff5967016a5f816b3bb)
- Add support for non-array types in dr.CustomOp
- Various minor fixes in C++ test suite [[dcaf69a]](https://github.com/mitsuba-renderer/drjit/commit/dcaf69a7a8531692146ef489506cff40b2fab34f)