glsl: Rename uniform_field_visitor to program_resource_visitor.

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.

NOTE: This is a candidate for the 9.1 branch.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>