Sample shading can be used to specify a minimum number of unique samples to
process for each fragment.
Sample shading is controlled by the sampleShadingEnable member of
VkPipelineMultisampleStateCreateInfo.
If sampleShadingEnable is VK_FALSE, sample shading is considered
disabled and has no effect.
Otherwise, an implementation must provide a minimum of max(⌈
minSampleShading × rasterizationSamples ⌉, 1)
unique associated data for each fragment, where minSampleShading is
the minimum fraction of sample shading and rasterizationSamples is the
number of samples requested in VkPipelineMultisampleStateCreateInfo.
These are associated with the samples in an implementation-dependent manner.
When the sample shading fraction is 1.0, a separate set of associated data
are evaluated for each sample, and each set of values is evaluated at the
sample location.