BSDF plugins in CentralPatchSurface were in the wrong order

[schunkes]

Description

The blendbsdf plugin in mitsuba sorts the nested BSDFs alphanumerically by their identifier. That means, that the previous names for the two BSDFs in the CentralPatchSurface would put them in the wrong order. The updated naming solves that issue.

Checklist

• The code follows the relevant coding guidelines
• The code generates no new warnings
• The code is appropriately documented
• The code is tested to prove its function
• The feature branch is rebased on the current state of the main branch
• I updated the change log if relevant
• I give permission that the Eradiate project may redistribute my contributions under the terms of its license