vdpau: don't crash on flipped images

It seems the vdpau API does not support these.

Do a semi-expensive emulation of it. On the other hand, it's not like
this is a commonly-used feature. (It might be better to make --vf=flip
always copy instead of flipping it via pointer tricks - but everything
allows flipped images, and even decoders or libavfilter filters could
output them.)
build: fix cross-compilation of libmpv

libmpv requires nm for creating the list of exported symbols (this is
done in syms.py). We should probably just make this list static instead,
but since this involves platform-specific code, for now this quick-fix
will do.