videorate: avoid useless buffer copy in drop-only mode

videorate: avoid useless buffer copy in drop-only mode

Make writable the buffer before pushing it lead to a buffer copy. It's
because a reference is keep for the previous buffer.
The previous buffer reference is only need to duplicate the buffer. In
drop-only mode, the previous buffer is release just after pushing the
buffer so a copy is done but it's useless.

https://bugzilla.gnome.org/show_bug.cgi?id=764319