Add Context::new_with_codec

Allows calling `avcodec_alloc_context3` with an AVCodec, setting the default values.

src/codec/context.rs

@@ -41,6 +41,15 @@ impl Context {
         }
     }
 
+    pub fn new_with_codec(codec: Codec) -> Self {
+        unsafe {
+            Context {
+                ptr: avcodec_alloc_context3(codec.as_ptr()),
+                owner: None,
+            }
+        }
+    }
+
     pub fn from_parameters<P: Into<Parameters>>(parameters: P) -> Result<Self, Error> {
         let parameters = parameters.into();
         let mut context = Self::new();