Apparently we need those to make sure that the initial layout
transition itself is valid. We don't need to add shader access
though, since we cannot access attachments inside shaders.
Fixes validation errors and some unnecessary cruft. We don't need
visibility operations in the external-to-renderpass dependency
because that part is handled by regular barriers, an execution
dependency is enough (but still required to handle write-after-read.
We have to use VK_IMAGE_LAYOUT_GENERAL for those. On top of that,
we should avoid image transitions when the image is in GENERAL
layout anyway in order to save some time on the GPU.
