Adjusted a few checks in the GraphicsPipeline constructor,
specifically stages' specialization info and sample mask,
which can either be null or default.

Also fixed destroying Pipeline objects, making sure there's
no confusion between GraphicsPipeline and ComputePipeline's
destroy() methods inherited from their 2 base classes
(Pipeline::destroy() and ObjectBase::destroy()).

Bug b/118386749

Change-Id: I78be9f0bbb735518e07d52f7372dbf08b9834abf
Reviewed-on: https://swiftshader-review.googlesource.com/c/23110Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>