testrender: Implement basic displacement shader support

[fpsunflower]

Description

Associate an (optional) displacement shader with each material. On startup, we execute the displacement for each face-vertex that has a valid displacement shader assigned.

Tests

Added a new test with a displaced sphere to show how this works.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• 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.

[fpsunflower]

Looks like I need to grab updated reference images from the CI machine.

In any case, this PR should be reviewed after we get #1897 merged.

[lgritz]

Update: I've merged #1897 now, so you ought to be able to rebase on top of it in main.