Vulkan: Crunch RenderPassDesc. This reduces the size of the RenderPass desc from 64 to 12 bytes. Bug: angleproject:2522 Change-Id: Iff2df87ba65be0bd976bba81c76c285cb0fa1ceb Reviewed-on: https://chromium-review.googlesource.com/c/1308459 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>