Instead of managing a GrowingSlice<> more or less shared between
several RenderPass, RenderPass now works directly with an Arena which
owns the commands. We do this so RenderPass can be copiable, which is
useful for creating a RenderPass from a "template" RenderPass
(usually empty, but with some scene parameters set).
RenderPass is now a lot more self contained.
Execution of the commands is now done with a RenderPass::Execcutor
which can be obtained from a RenderPass.
This change led to some code clean-up as well.
We also had to improve Arena<> to be able to take a custom Area.
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