Move jpeg_finish_compress out of destructor (#433)

* Move jpeg_finish_compress out of destr

Moving the function out will ensure
that no function that can error will
be called inside a destructor, thus
leaving no chance to get two exceptions
in flight or finishing compression even
if an error occured

* Comments and minor stylistical changes

Added comments to explain why finish
compress was moved out of destructor,
and did stylistical changes

include/boost/gil/extension/io/jpeg/detail/write.hpp

@@ -132,6 +132,8 @@ class writer< Device
                                 , 1
                                 );
         }
+
+        jpeg_finish_compress ( this->get() );
     }
 };
 

include/boost/gil/extension/io/jpeg/detail/writer_backend.hpp

@@ -113,7 +113,9 @@ struct writer_backend< Device
 
     ~writer_backend()
     {
-        jpeg_finish_compress ( get() );
+        // JPEG compression object destruction does not signal errors,
+        // unlike jpeg_finish_compress called elsewhere,
+        // so there is no need for the setjmp bookmark here.
         jpeg_destroy_compress( get() );
     }