Context time_base / frame_rate adjustments

- Moves `time_base` from `Decoder` and `set_time_base` from `Encoder` to the base `Context` to unify the API for both types.
- Adds `packet_time_base` and `set_packet_time_base` to `Decoder`.
- Moves `set_frame_rate` from `Encoder` to `Context` and adds a `frame_rate` getter.

src/codec/context.rs

@@ -8,7 +8,7 @@ use super::{threading, Compliance, Debug, Flags, Id, Parameters};
 use ffi::*;
 use libc::c_int;
 use media;
-use {Codec, Error};
+use {Codec, Error, Rational};
 
 pub struct Context {
     ptr: *mut AVCodecContext,
@@ -138,6 +138,31 @@ impl Context {
             }
         }
     }
+
+    pub fn time_base(&self) -> Rational {
+        unsafe { Rational::from((*self.as_ptr()).time_base) }
+    }
+
+    pub fn set_time_base<R: Into<Rational>>(&mut self, value: R) {
+        unsafe {
+            (*self.as_mut_ptr()).time_base = value.into().into();
+        }
+    }
+
+    pub fn frame_rate(&self) -> Rational {
+        unsafe { Rational::from((*self.as_ptr()).framerate) }
+    }
+
+    pub fn set_frame_rate<R: Into<Rational>>(&mut self, value: Option<R>) {
+        unsafe {
+            if let Some(value) = value {
+                (*self.as_mut_ptr()).framerate = value.into().into();
+            } else {
+                (*self.as_mut_ptr()).framerate.num = 0;
+                (*self.as_mut_ptr()).framerate.den = 1;
+            }
+        }
+    }
 }
 
 impl Default for Context {

src/codec/decoder/decoder.rs

@@ -107,8 +107,14 @@ impl Decoder {
         }
     }
 
-    pub fn time_base(&self) -> Rational {
-        unsafe { Rational::from((*self.as_ptr()).time_base) }
+    pub fn packet_time_base(&self) -> Rational {
+        unsafe { Rational::from((*self.as_ptr()).pkt_timebase) }
+    }
+
+    pub fn set_packet_time_base<R: Into<Rational>>(&mut self, value: R) {
+        unsafe {
+            (*self.as_mut_ptr()).pkt_timebase = value.into().into();
+        }
     }
 }
 

src/codec/encoder/encoder.rs

@@ -116,23 +116,6 @@ impl Encoder {
             }
         }
     }
-
-    pub fn set_time_base<R: Into<Rational>>(&mut self, value: R) {
-        unsafe {
-            (*self.as_mut_ptr()).time_base = value.into().into();
-        }
-    }
-
-    pub fn set_frame_rate<R: Into<Rational>>(&mut self, value: Option<R>) {
-        unsafe {
-            if let Some(value) = value {
-                (*self.as_mut_ptr()).framerate = value.into().into();
-            } else {
-                (*self.as_mut_ptr()).framerate.num = 0;
-                (*self.as_mut_ptr()).framerate.den = 1;
-            }
-        }
-    }
 }
 
 impl Deref for Encoder {