The SPIRV previously checked in had various issues -- most commonly, it
would use the VertexId and InstanceId builtins rather than the
replacmenet VertexIndex and InstanceIndex builtins required by Vulkan.
The sample count was previously fixed at 8, which is not supported
on some platforms. The code now checks for the maximum supported
sample count and uses that instead.
If you would prefer the upper limit to be fixed at 8, that is
obviously trivial to change.
