Layer closure implementation in testrender

[AdrienHerubel]

Description

This PR provides an implementation of the layer() closure as described in stdosl.h :

// Vertically layer a layerable BSDF such as dielectric_bsdf, generalized_schlick_bsdf or
// sheen_bsdf over a BSDF or VDF. The implementation is target specific, but a standard way
// of handling this is by albedo scaling, using "base*(1-reflectance(top)) + top", where
// reflectance() calculates the directional albedo of a given top BSDF.

Tests

I added render-mx-layer which exercises nested layer closures, with one fully opaque top layer.

Checklist:

• [x ] I have read the contribution guidelines.
• [ x] I have previously submitted a Contributor License Agreement.
• [ 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).
• My code follows the prevailing code style of this project.

[fpsunflower]

This looks great! Thanks!