Auto merge of #30 - pcwalton:retain-release-pixel-format, r=jdm

Add bindings to `CGLRetainPixelFormat` and `CGLReleasePixelFormat`

r? @jdm

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src/cgl.rs

@@ -159,6 +159,9 @@ extern {
                                   attrib: CGLPixelFormatAttribute,
                                   value: *mut GLint) -> CGLError;
     pub fn CGLDestroyPixelFormat(pix: CGLPixelFormatObj) -> CGLError;
+    pub fn CGLReleasePixelFormat(pix: CGLPixelFormatObj);
+    pub fn CGLRetainPixelFormat(pix: CGLPixelFormatObj) -> CGLPixelFormatObj;
+    pub fn CGLGetPixelFormatRetainCount(pix: CGLPixelFormatObj) -> GLuint;
 
     // Context functions
     pub fn CGLCreateContext(pix: CGLPixelFormatObj, share: CGLContextObj, ctx: *mut CGLContextObj) ->