Vulkan: Mask out Depth/Stencil RTs in feedback loops. This should enable some cases of limited support for feedback loops with depth/stencil buffers. For example with Manhattan and the Vulkan back-end. Increases the number of RenderPasses in Manhattan slightly. This will regress performance slightly until we can work out a better solution that is also conformant with the spec. Bug: angleproject:4517 Change-Id: I2758e6b4c2a930474c09cdc0950f3b6c34541089 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2106670 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>