feat(codec): allow codec context creation with specified codec

For some codecs (x264) it's necessary to set the codec while
the context is being created, otherwise the codec-specific defaults
aren't applied.

src/codec/context.rs

@@ -38,6 +38,15 @@ impl Context {
 		}
 	}
 
+	pub fn for_codec(codec: &Codec) -> Self {
+		unsafe {
+			Context {
+				ptr: avcodec_alloc_context3(codec.as_ptr()),
+				owner: None,
+			}
+		}
+	}
+
 	pub fn decoder(self) -> Decoder {
 		Decoder(self)
 	}

src/codec/decoder/decoder.rs

@@ -13,6 +13,13 @@ use crate::{
 pub struct Decoder(pub Context);
 
 impl Decoder {
+	pub fn new<D: traits::Decoder>(codec: D) -> Result<Self, Error> {
+		let codec = codec.decoder().ok_or(Error::DecoderNotFound)?;
+		let context = Context::for_codec(&codec);
+
+		Ok(Decoder(context))
+	}
+
 	pub fn open(mut self) -> Result<Opened, Error> {
 		unsafe {
 			match avcodec_open2(self.as_mut_ptr(), ptr::null(), ptr::null_mut()) {

src/codec/encoder/encoder.rs

@@ -5,11 +5,22 @@ use std::{
 };
 
 use super::{audio, subtitle, video};
-use crate::{codec::Context, ffi::*, media, packet, Error, Frame, Rational};
+use crate::{
+	codec::{traits, Context},
+	ffi::*,
+	media, packet, Error, Frame, Rational,
+};
 
 pub struct Encoder(pub Context);
 
 impl Encoder {
+	pub fn new<E: traits::Encoder>(codec: E) -> Result<Self, Error> {
+		let codec = codec.encoder().ok_or(Error::EncoderNotFound)?;
+		let context = Context::for_codec(&codec);
+
+		Ok(Encoder(context))
+	}
+
 	pub fn video(mut self) -> Result<video::Video, Error> {
 		match self.medium() {
 			media::Type::Unknown => {