vdpau.h: change vdpau_render_state layout to match fork if AV_HAVE_INCOMPATIBLE_FORK_ABI

This might fix a compatibility issue.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
error_resilience: fix const correctness, silence warnings

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