This CL does the following:
* Adds a "transient attachment" bit to texture usage. This bit
specifies that the created texture will be used only during
this render pass.
* Adds a TransientAttachments Feature that gates the usage of
transient attachments.
* Adds support for transient attachments on Metal, where they're
used to create textures as memoryless.
* Adds validation tests and an E2T test of the feature.
A followup CL will add support in Vulkan.
Bug: dawn:1695
Change-Id: I3c7322dd1e4bee113062aae2e0494d292ee8cbc3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/129080
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
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