This removes a lot of code and will make it easier to migrate into a new
library with Java bindings, etc.
- The clipping planes and projection matrix were unused, removed them.
- In split view mode, our demos were using only 2 of the 3 manipulators,
removed one.
- Instead of callbacks and update methods, users can simply ask for the
current matrix at every frame.
- The manipulator's job is to consume mouse events and maintain an
"look at" basis, there is no need to store a reference to a Filament
Camera.
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