Adding a getPlane() function in order to not have the "plane" private
member unused, which causes a warning and makes compilation fail.

Change-Id: I3ec08d3439c7e8f3d4314f3b4864d12e9c34d6f4
Reviewed-on: https://swiftshader-review.googlesource.com/18468Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>