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.
