We need to construct the output format list separatedly from the input
format list, because we need to adhere to two extra requirements:

1. Big-endian output formats are always unsupported (runtime error)
2. Combining 'vulkan' with an explicit out_format that is not supported
   by the vulkan frame allocation code is illegal and will crash (abort)

As a free side benefit, this rewrite fixes a possible memory leak in the
`fail` path that was present in the old code.
Signed-off-by: Niklas Haas <git@haasn.dev>