Inverse model transform matrix in shaders

[pbaja]

I am creating a shader in which I need to convert the camera position from world space to object space.
There is access to the model matrix via mesh.model but there is no inverse equivalent (only inversed & transposed).
Unity, for example, provides it via the unity_WorldToObject variable.

I have two questions:

1. If I would like to add it to my game, where/how should I do that when I need a variable updated before rendering, per mesh? (The per mesh part is troubling to me)
2. Will it make sense to add it to the engine, even if it's not being used by the engine? Just to make it easier for users to write custom shaders? Like Unity and other game engines do.

[pbaja]

As this is a trivial change, I have created a pull request: #6608