qtdemux: fix wrong full_range offset when parsing colr box

use colr_data[18] >> 7 to get full range information, instead of colr_data[17] >> 7

following hex code is a colr box example, the offset of 0x80 is 18

00 00 00 13  63 6F 6C 72  6E 63 6C 78  00 01 00 01   00 00 80