Some shaders can be shared across all materials (e.g. the depth
shaders). We use the filament default material as the "source" of
the cache, but until now we relied on an a priori knowledge of which
variants were present in the default material.
With this change, we now query once the list of variants (of interest)
in the default material and reuse that list for caching these variants
later.
This is better because the cached variants are now entirely driven by
the default material (which they depend on anyways). This is also faster
because we don't need to query which variant we need each time we create
a material.
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