fix(batched): Assume BatchedRendererServices texture derivatives are in st space.

[sfriedmapixar]

The convention in the single-point RendererServices is that the texture call returns derivatives in st space, and they are transformed to xy space before returning from the wrapper to RenderServices. This change makes BatchedRendererServices follow the same convention.

This is still tested by testrender/testshade, but the code motion means the assumptions visible at the renderer/OSL boundary are now consistent between batched/non-batched renderservices.

Checklist:

• [X] I have read the contribution guidelines.
• [X] I have updated the documentation, if applicable.
• [X] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [X] My code follows the prevailing code style of this project. If I haven't already run clang-format v17 before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.