Caption support (#2462)

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

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

@@ -0,0 +1,29 @@
+package astits
+
+import (
+	"time"
+)
+
+// ClockReference represents a clock reference
+// Base is based on a 90 kHz clock and extension is based on a 27 MHz clock
+type ClockReference struct {
+	Base, Extension int64
+}
+
+// newClockReference builds a new clock reference
+func newClockReference(base, extension int64) *ClockReference {
+	return &ClockReference{
+		Base:      base,
+		Extension: extension,
+	}
+}
+
+// Duration converts the clock reference into duration
+func (p ClockReference) Duration() time.Duration {
+	return time.Duration(p.Base*1e9/90000) + time.Duration(p.Extension*1e9/27000000)
+}
+
+// Time converts the clock reference into time
+func (p ClockReference) Time() time.Time {
+	return time.Unix(0, p.Duration().Nanoseconds())
+}