glsl: optimise dead built-in varyings in core profile
This code to skip the built-in optimisations in core profile has existed since this code was first committed around 10 years ago. However the assumption that shaders used in core profile don't contain built-ins has always been wrong.
There are a couple of ways a shader used in a core profile can contain one of these built-in varyings.
- The core profile uses shaders with an old shader version.
- The core profile uses shaders that enable compat shader functionality explicitly even in the latest shader versions.
Rather than adding any new code to track if we are using a compat shader we simply enable the optimisation to attempt to run on all desktop shaders.