This is a prep step for the upcoming morph feature and does not require
a bump to our material version number.
Stay tuned for a new sample app that demonstrates this feature.
This replaces the previous "curvature to roughness" method. Both are related
and rely on the screen space variance of geometric normals but this new
solution offers more control (the screen space variance and the clamping
threshold can be controlled).
In the public docs and materials API, Filament "variables" are custom
interpolants (outputs of VS, inputs of FS). However we internally used
"variables" to also refer to built-in VS inputs.
CodeGenerator had a generateVariable method and a generateVariables
method, which did something completely different. :)
