glsl: reject samplers not declared as uniform/function params earlier

This improves consistency with image variables and atomic
counters which are already rejected the same way.

Note that opaque variables can't be treated as l-values, which
means only the 'in' function parameter is allowed.

v2: rewrite commit message

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v2)

src/compiler/glsl/ast_to_hir.cpp

@@ -3589,6 +3589,15 @@ apply_layout_qualifier_to_variable(const struct ast_type_qualifier *qual,
       }
    }
 
+   if (var->type->contains_sampler()) {
+      if (var->data.mode != ir_var_uniform &&
+          var->data.mode != ir_var_function_in) {
+         _mesa_glsl_error(loc, state, "sampler variables may only be declared "
+                          "as function parameters or uniform-qualified "
+                          "global variables");
+      }
+   }
+
    /* Is the 'layout' keyword used with parameters that allow relaxed checking.
     * Many implementations of GL_ARB_fragment_coord_conventions_enable and some
     * implementations (only Mesa?) GL_ARB_explicit_attrib_location_enable