This directive requires specific shader models to be tested for
compilation, bypassing the default behaviour where only one version from
each shader model set (SM1-3, SM4-5, SM6) is compiled.
Mostly to be able to associate a version number to each tag and
get rid of all the foo<1.2.3 tags. The new system also has fixed
tag slots, rather than dealing with strings, so we don't have to
manually adjust the size of the `tags' array.
With the new system each tag can be present or not, and if it is
present it can have an associated version number (of the form
major.minor.patch). If the version is not available, it is set to
0.0.0. Each tag can be queried for existence and for comparison
with the version number.
So we have a more recent version of SPIRV-Tools and also don't
have to recompile Mesa to test llvmpipe. This fixes a few failing
tests, but also breaks a couple.
Similar to how we have the "geometry-shader" cap. In principle shader
model 5+ implies support for tessellation shaders, but the Vulkan,
OpenGL, and Metal runners are able to support most of shader model 5+
without the underlying GPU (or API) necessarily supporting tessellation
shaders.
