format: add support for Chapters

Read chapters from an input context and add chapters to an output context.

Note: unlike avformat_new_stream, the equivalent function for chapter is private:
avpriv_new_chapter (part of libavformat/internal.h). I couldn't find any other
solution but re-implementing it in format::context::output::add_chapter.

src/format/context/output.rs

@@ -1,9 +1,12 @@
 use std::ops::{Deref, DerefMut};
 use std::ptr;
+use std::mem::size_of;
 use std::ffi::CString;
 
+use libc;
+
 use ffi::*;
-use ::{Error, StreamMut, Dictionary, format};
+use ::{Error, StreamMut, ChapterMut, Rational, Dictionary, format};
 use super::common::Context;
 use super::destructor;
 use codec::traits;
@@ -81,6 +84,65 @@ impl Output {
 		}
 	}
 
+	pub fn add_chapter<R: Into<Rational>, S: AsRef<str>>(&mut self,
+		id: i32,
+		time_base: R,
+		start: i64,
+		end: i64,
+		title: S) -> Result<ChapterMut, Error>
+	{
+		// avpriv_new_chapter is private (libavformat/internal.h)
+
+		if start > end {
+			return Err(Error::InvalidData);
+		}
+
+		let mut existing = None;
+		for chapter in self.chapters() {
+			if chapter.id() == id {
+				existing = Some(chapter.index());
+				break;
+			}
+		}
+
+		let index = match existing {
+			Some(index) => index,
+			None => unsafe {
+				let     ptr         = av_mallocz(size_of::<AVChapter>()).as_mut().ok_or(Error::Bug)?;
+				let mut nb_chapters = (*self.as_ptr()).nb_chapters as i32;
+
+				// chapters array will be freed by `avformat_free_context`
+				av_dynarray_add(
+					&mut (*self.as_mut_ptr()).chapters as *mut _ as *mut libc::c_void,
+					&mut nb_chapters,
+					ptr
+				);
+
+				if nb_chapters > 0 {
+					(*self.as_mut_ptr()).nb_chapters = nb_chapters as u32;
+					let index = (*self.ctx.as_ptr()).nb_chapters - 1;
+					index as usize
+				}
+				else {
+					// failed to add the chapter
+					av_freep(ptr);
+					return Err(Error::Bug);
+				}
+			},
+		};
+
+		let mut chapter = self.chapter_mut(index)
+			.ok_or(Error::Bug)?;
+
+		chapter.set_id(id);
+		chapter.set_time_base(time_base);
+		chapter.set_start(start);
+		chapter.set_end(end);
+		chapter.set_metadata("title", title);
+
+		Ok(chapter)
+	}
+
 	pub fn set_metadata(&mut self, dictionary: Dictionary) {
 		unsafe {
 			(*self.as_mut_ptr()).metadata = dictionary.disown();