Index-/VertexBuffer from FilamentAsset

[27justin]

Is your feature request related to a problem? Please describe.
AFAIK the default createInstancedAsset / createInstance functions that FilamentAsset provides may not be sufficient for use-cases where the user requires GPU instancing.

Ideally I'd want Filament to expose the VertexBuffer(s) and IndexBuffer(s) of a given asset transparently to the user, so that he can build a Renderable with RenderableBuilder::Builder::instances(size_t) to render large quantities of the same asset over a scene.

Describe the solution you'd like
I'm currently using a patched version of Filament that exposes two new functions on FilamentAsset:

	const std::vector<VertexBuffer*> &getVertexBuffers() const noexcept;
	const std::vector<IndexBuffer*> &getIndexBuffers() const noexcept;

Describe alternatives you've considered
I tried using the createInstancedAsset function as well as createInstance to render ~20.000 blades of grass.
However the performance was ~10-20x worse than using the RenderableBuilder::Builder::instances(size_t) approach.

OS and backend
N/A