This switches where we notify the Texture class that the back-end has
a dirty staging buffer. Previously we notified in every possible code
site in TextureVk. Now we notify in a single centralized location in
the staging buffer. This will allow us to stash clears in the staging
buffer in the FramebufferVk class via RenderTargetVk. And ultimately
let us defer clears to fix Manhattan's rendering feedback loop.

Bug: angleproject:4517
Change-Id: I67760f133e2c58feddbba3d1062de0b966bac281
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2128546Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>