gsk/vulkan/image: Use UNDEFINED for initial layout

gsk/vulkan/image: Use UNDEFINED for initial layout

When creating an image using gsk_vulkan_image_new_for_framebuffer(),
it passes VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL.

However, this is a mistake. The spec demands that the initial
layout must be either VK_IMAGE_LAYOUT_UNDEFINED or
VK_IMAGE_LAYOUT_PREINITIALIZED.

Apparently this was an oversight from commit b97fb751469, since the
commit message even documents that, and all other calls pass either
VK_IMAGE_LAYOUT_UNDEFINED or VK_IMAGE_LAYOUT_PREINITIALIZED.

Create framebuffer images using VK_IMAGE_LAYOUT_UNDEFINED, which is
what was originally expected.