Vulkan: Encapsulate RenderTargetVk. This makes the members private and adds more functionality. This moves more responsibility out of vk::CommandGraphNode and also makes the RenderPass init in the CommandGraphNode class better encapsulated. Bug: angleproject:2539 Change-Id: Ia16f3f39cf011548c6473805b8b28e284808e856 Reviewed-on: https://chromium-review.googlesource.com/1040279 Reviewed-by: Luc Ferron <lucferron@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>