Vulkan: Only init RenderPass once per frame. This saves some time spent in the driver, by making multiple draw calls happen inside a single RenderPass. This also makes the ReadPixels impl method non-const. I think in the future we should avoid making const Impl methods unless they're totally trivial. BUG=angleproject:1898 Change-Id: I39172270a2f7dc5c1c2e3d4cc50af3bac8a29fa1 Reviewed-on: https://chromium-review.googlesource.com/672148 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>