The Mesa shader cache core support and drivers prevent shader cache
usage when a shader program uses transform feedback.
We should try to remove this restriction for performance reasons,
and to simplify some code paths, by removing the need to check
for transform feedback.
From irc, Tim mentions that we need to add
prog->TransformFeedback->VaryingNames into the sha1 in
shader_cache_read_program_metadata. Similar to
Should be fixed by these commits:
glsl/shader_cache: Allow shader cache usage with transform feedback
i965: Allow disk shader cache usage with transform feedback