lavfi/buffersrc: remove unused item from BufferSourceContext

Signed-off-by: Paul B Mahol <onemda@gmail.com>
lavfi: add attribute_align_arg to all public entry points

Previously it was partly only added to central functions called
internally, however if GCC would inline these into the public fuction, the
alignment attribute would not take effect.

Instead, add it to all public entry points to avoid these problems.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>