Data is passed into and out of shaders using variables with input or output
storage class, respectively.
User-defined inputs and outputs are connected between stages by matching
their Location decorations.
Additionally, data can be provided by or communicated to special functions
provided by the execution environment using BuiltIn decorations.
In many cases, the same BuiltIn decoration can be used in multiple
shader stages with similar meaning.
The specific behavior of variables decorated as BuiltIn is documented
in the following sections.