Add new image for libplacebo based on ubuntu cosmic
We use ubuntu rather than debian because the former has a package for the vulkan validation layers whereas the latter appears not to. Unfortunately, even they don't support libshaderc, but we've got to work with what we've got - so we install it by hand. (Taking care not to shit all over the glslang package's contents) Incidentally, while I install the mesa vulkan drivers, they probably won't actually work in the test suite unless we actually have any GPU inside the VM. But that's a separate concern.
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