.length() on an unsized SSBO variable doesn't actually read any data from the SSBO, and is allowed on variables marked 'writeonly'. Fixes compute shader compilation in Shadow of Mordor. Signed-off-by: Kenneth Graunke <kenneth@whitecape.org> Reviewed-by: Iago Toral Quiroga <itoral@igalia.com> Reviewed-by: Matt Turner <mattst88@gmail.com>