Added max to bitmap and checkerboard textures, added tests for both m…

[Microno95]

Description

Adds implementations of max for the bitmap and checkerboard textures, adds python binding and mean/max tests for vectorised variants. Fixes #707

Testing

Ran entire python test suite in Release mode.

Checklist

• My code follows the style guidelines of this project
• My changes generate no new warnings
• My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• I have commented my code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I cleaned the commit history and removed any "Merge" commits
• I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license

[njroussel]

I have a few comments (they are also true for the rebuild_internals method).

The suggestions might not actually be applicable/reasonable for your initial use case (volumes), I hadn't considered this when you opened the issue. I don't want to discourage using the Texture interface for volumes, but I don't think the Bitmap plugin is the appropriate choice.

[Microno95]

Hey @njroussel, have you had a chance to read any of my comments?

[Microno95]

I've updated the implementation to use the luminance rather than the component-wise maximum for both the RGB and Spectral cases.