Read JPEG markers before, between, or after compressed-data scans. Change state as necessary when a new scan is reached. Return value is JPEG_SUSPENDED, JPEG_REACHED_SOS, or JPEG_REACHED_EOI. The consume_input method pointer points either here or to the coefficient controller's consume_data routine, depending on whether we are reading a compressed data segment or inter-segment markers. Note: This function should NOT return a pseudo SOS marker(with zero component number) to the caller.A pseudo marker received by read_markers is processed and then skipped for other markers.