glsl: Support transform feedback of varying structs.

Since transform feedback needs to be able to access individual fields
of varying structs, we can no longer match up the arguments to
glTransformFeedbackVaryings() with variables in the vertex shader.

Instead, we build up a hashtable which records information about each
possible name that is a candidate for transform feedback, and then
match up the arguments to glTransformFeedbackVaryings() with the
contents of that hashtable.

Populating the hashtable uses the program_resource_visitor
infrastructure, which so the logic is shared with how we handle
uniforms.