glsl: Rename uniform_field_visitor to program_resource_visitor.

There's actually nothing uniform-specific in uniform_field_visitor.
It is potentially useful for all kinds of program resources (in
particular, future patches will use it for transform feedback
varyings).

This patch renames it to program_resource_visitor, and clarifies
several comments, to reflect the fact that it is useful for more than
just uniforms.
WIP: use shared code in tfeedback_decl::init()
Use a hashtable to keep track of transform feedback candidates