vd_lavc: when falling back to software, revert filter error status

When mpv is started with some video filters set (--vf is used), and
hardware decoding is requested, and hardware decoding would be possible,
but is prevented due to video filters that accept software formats only,
the fallback didn't work properly sometimes.

This fallback works rather violently: it tries to initialize the filter
chain, and if it fails it throws away the frame decoded using the
hardware, and retries with software. The case that didn't work was when
decoding the current packet didn't immediately lead to a new frame. Then
the filter chain wouldn't be reinitialized, and the playloop would stop
playback as soon as it encounters the error flag.

Fix this by resetting the filter error flag (back to "uninitialized"),
which is a rather violent, but somewhat working solution.

The fallback in general should perhaps be cleaned up later.