glsl: Allow gl_nir_lower_samplers*() without a gl_shader_program

I would like to be able to run gl_nir_lower_samplers() to turn texture
and sampler variable dereferences into indexes and offsets, even for
ARB programs, and built-in shaders.  This would make sampler handling
more consistent across the various types of shaders.

For GLSL programs, the gl_nir_lower_samplers_as_deref() pass looks up
the variable bindings in the shader program's uniform storage.  But
ARB programs and built-in shaders don't have a gl_shader_program, and
uniform storage doesn't exist.  In this case, we simply skip that
lookup, and trust var->data.binding to be set correctly by whoever
created the shader.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/glsl/gl_nir_lower_samplers_as_deref.c

@@ -147,10 +147,18 @@ lower_deref(nir_builder *b, struct lower_samplers_as_deref_state *state,
 
    remove_struct_derefs_prep(path.path, &name, &location, &type);
 
+   if (state->shader_program) {
+      /* For GLSL programs, look up the bindings in the uniform storage. */
       assert(location < state->shader_program->data->NumUniformStorage &&
              state->shader_program->data->UniformStorage[location].opaque[stage].active);
 
       binding = state->shader_program->data->UniformStorage[location].opaque[stage].index;
+   } else {
+      /* For ARB programs or built-in shaders, assume that whoever created
+       * the shader set the bindings correctly already.
+       */
+      binding = var->data.binding;
+   }
 
    if (var->type == type) {
       /* Fast path: We did not encounter any struct derefs. */