glsl_to_nir: zero-initialize var->data.descriptor_set

This is convenient for backends that support both Vulkan and OpenGL while
lowering samplers to derefs with nir_lower_samplers_as_deref.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/glsl_to_nir.cpp

@@ -424,6 +424,7 @@ nir_visitor::visit(ir_variable *ir)
    }
 
    var->data.index = ir->data.index;
+   var->data.descriptor_set = 0;
    var->data.binding = ir->data.binding;
    var->data.offset = ir->data.offset;
    var->data.image.read_only = ir->data.memory_read_only;