Define layer pass-through semantics

[tdavidovicNV]

While reviewing the layer equation added in #2964, I found that deriving base attenuation as 1 - E_top conflates two different outcomes: energy absorbed by the top layer and energy passed toward the base.

This is visible with a colored reflection-only dielectric. For untinted Fresnel albedo F = 0.04 and tint c = (1, 0.25, 0.05), the complete top albedo is:

E_top = c * F = (0.04, 0.01, 0.002)

The current specification therefore gives base attenuation (0.96, 0.99, 0.998). The existing GLSL and OSL testrender implementations instead use the untinted unreflected share (1 - F) = (0.96, 0.96, 0.96), treating the tint-removed energy as absorption rather than returning it to the base.

This PR introduces an explicit color-valued pass-through factor that may depend on both incident and outgoing directions:

$$P_{\mathrm{top}}(\omega_i, \omega_o) \in [0, 1]$$ $$f(\omega_i, \omega_o) = f_{\mathrm{top}}(\omega_i, \omega_o) + P_{\mathrm{top}}(\omega_i, \omega_o) f_{\mathrm{base}}(\omega_i, \omega_o)$$

This follows the two-leg construction in Weidlich and Wilkie 2007, section 3.2, where the lower-layer BRDF is attenuated by entry transmission, absorption over the incident and outgoing paths, and return transmission.

For nested layers, the pass-through factors multiply recursively. The PR also separates BSDF-over-BSDF composition from a BSDF bound to a VDF: a lossless transmissive interface has complete directional albedo near one, but that should not prevent transmitted light from entering the medium.

The current implementations are not changed:

• GLSL: Approximates pass-through using only the outgoing direction, stores it in BSDF.throughput, and multiplies it in layer. Reflection-only dielectric and sheen keep artistic color out of pass-through, while generalized Schlick reduces its color-valued albedo to an RGB average.
• OSL (testrender): Also approximates pass-through using only the outgoing direction. It computes top opacity from filter_o and scales the base by one minus that value. Dielectric uses untinted pass-through, sheen uses a scalar estimate based on the maximum color component, and generalized Schlick uses a scalar energy estimate.
• MDL: Rewrites a compatible subset of layer graphs into native layerers. Its curve layerers evaluate both directions and use a symmetric maximum. MaterialX sheen uses a scalar weighted-layer approximation based on average color. Unsupported top nodes cannot retain the base.