fix(codec, format)!: detect unset time_bases, allow unsetting time_base

Instead of silently passing possibly unset `time_base`s back into ffmpeg,
force users to explicitly handle (even if just by early panic) their absence.

BREAKING CHANGE: `time_base` and `set_time_base` functions now
return/accept Option<Rational> (±Into). `unwrap`/`expect` is a valid way
to surface this unwanted state early in user code.

For symmetry, `set_time_base` also allows unsetting by passing a `None`,
so all existing calls (with non-zero timebases) need to wrap the
argument in `Some`.

src/codec/context.rs

@@ -125,13 +125,13 @@ impl Context {
 		Parameters::from(self)
 	}
 
-	pub fn time_base(&self) -> Rational {
-		unsafe { Rational::from((*self.as_ptr()).time_base) }
+	pub fn time_base(&self) -> Option<Rational> {
+		unsafe { Rational::from((*self.as_ptr()).time_base).non_zero() }
 	}
 
-	pub fn set_time_base<R: Into<Rational>>(&mut self, value: R) {
+	pub fn set_time_base<R: Into<Rational>>(&mut self, value: Option<R>) {
 		unsafe {
-			(*self.as_mut_ptr()).time_base = value.into().into();
+			(*self.as_mut_ptr()).time_base = value.map(Into::into).unwrap_or(Rational::ZERO).into();
 		}
 	}
 

src/format/chapter/chapter.rs

@@ -26,8 +26,8 @@ impl<'a> Chapter<'a> {
 		unsafe { (*self.as_ptr()).id }
 	}
 
-	pub fn time_base(&self) -> Rational {
-		unsafe { Rational::from((*self.as_ptr()).time_base) }
+	pub fn time_base(&self) -> Option<Rational> {
+		unsafe { Rational::from((*self.as_ptr()).time_base).non_zero() }
 	}
 
 	pub fn start(&self) -> i64 {

src/format/stream/stream.rs

@@ -53,8 +53,8 @@ impl<'a> Stream<'a> {
 		unsafe { (*self.as_ptr()).index as usize }
 	}
 
-	pub fn time_base(&self) -> Rational {
-		unsafe { Rational::from((*self.as_ptr()).time_base) }
+	pub fn time_base(&self) -> Option<Rational> {
+		unsafe { Rational::from((*self.as_ptr()).time_base).non_zero() }
 	}
 
 	pub fn start_time(&self) -> Option<i64> {

src/format/stream/stream_mut.rs

@@ -26,9 +26,9 @@ impl<'a> StreamMut<'a> {
 }
 
 impl<'a> StreamMut<'a> {
-	pub fn set_time_base<R: Into<Rational>>(&mut self, value: R) {
+	pub fn set_time_base<R: Into<Rational>>(&mut self, value: Option<R>) {
 		unsafe {
-			(*self.as_mut_ptr()).time_base = value.into().into();
+			(*self.as_mut_ptr()).time_base = value.map(Into::into).unwrap_or(Rational::ZERO).into();
 		}
 	}