Caption support (#2462)

Co-authored-by: WithoutPants <53250216+WithoutPants@users.noreply.github.com>

vendor/github.com/asticode/go-astits/crc32.go

@@ -0,0 +1,25 @@
+package astits
+
+const (
+	crc32Polynomial = uint32(0xffffffff)
+)
+
+// computeCRC32 computes a CRC32
+// https://stackoverflow.com/questions/35034042/how-to-calculate-crc32-in-psi-si-packet
+func computeCRC32(bs []byte) uint32 {
+	return updateCRC32(crc32Polynomial, bs)
+}
+
+func updateCRC32(crc32 uint32, bs []byte) uint32 {
+	for _, b := range bs {
+		for i := 0; i < 8; i++ {
+			if (crc32 >= uint32(0x80000000)) != (b >= uint8(0x80)) {
+				crc32 = (crc32 << 1) ^ 0x04C11DB7
+			} else {
+				crc32 = crc32 << 1
+			}
+			b <<= 1
+		}
+	}
+	return crc32
+}