render/vulkan: don't use UNDEFINED layout for imported DMA-BUFs
UNDEFINED when used as source layout means that the contents of the underlying memory becomes undefined. This isn't what we want here: we don't want to mutate the imported pixel data.
The Vulkan spec isn't really clear what the proper value should be here, but after discussing with driver developers 1 it seems like UNDEFINED isn't the right one. The recommendation is to use GENERAL instead.