Using an initial layout of VK_IMAGE_LAYOUT_UNDEFINED together with
VK_ATTACHMENT_LOAD_OP_LOAD means that the render pass starts off with
completely undefined contents. Since we want to draw the overlay on top
of the previously rendered image, we want to use a real layout. Since
the calling code has already transitioned to VK_IMAGE_LAYOUT_PRESENT_SRC_KHR,
that's the layout we need to use.
This was never caught by validation because VK_IMAGE_LAYOUT_UNDEFINED is
the one layout you're allowed to use as an initial layout at any time
regardless of the image's current layout.
