Fix uv derivatives in testrender

[lgritz]

[bisqq]

I can take a look at this for dev days!

[lgritz]

@fpsunflower do you remember what this issue was about?

[fpsunflower]

Yes, the derivatives for u and v are not setup correctly in testrender. This makes it impossible to do certain common operations like bump mapping from a texture (I believe we have a test that does it through world space noise). It also means our texture filtering is not working properly.

It should just be a matter of implementing the chain rule properly. Bonus points if someone wants to implement a version that gives continuous gradients over triangle meshes (this may require storing the average gradient per vertex).

[lgritz]

Echoed from the slack channel (helpful reference contained):

Chris Kulla
Today at 8:15 AM
The goal will be to setup dudx,dudy,dvdx,dvdy correctly in testrender - it should be relatively straightforward. You can look at the derivation in https://graphics.stanford.edu/papers/trd/trd.pdf as an example