zink: return 256 for PIPE_CAP_MIN_MAP_BUFFER_ALIGNMENT
this isn't the minimum allowed by the driver, but zink doesn't return the minimum allowed by the driver anyway and hasn't in a very long time
instead, it suballocates using a minimum alignment of 256 bytes, so use that instead
fixes (in caselists): KHR-GL46.map_buffer_alignment.functional