Handle redeclarations

[AbigailBuccaneer]

All variable names, structure type names, and function names in a given scope share the same name space. Function names can be redeclared in the same scope, with the same or different parameters, without error. An implicitly-sized array can be redeclared in the same scope as an array of the same base type. Otherwise, within one compilation unit, a declared name cannot be redeclared in the same scope; doing so results in a redeclaration compile-time error. If a nested scope redeclares a name used in an outer scope, it hides all existing uses of that name. There is no way to access the hidden name or make it unhidden, without exiting the scope that hid it.

— GLSL 4.5 specification, §4.2 Scoping

We should detect redeclaration errors, as well as link valid declarations together.