vdpau: add AV_HWACCEL_FLAG_IGNORE_LEVEL to skip the codec level check

Decoding acceleration may work even if the codec level is higher than
the stated limit of the VDPAU driver. Or the problem may be considered
acceptable by the user. This flag allows skipping the codec level
capability checks and proceed with decoding.

Applications should obviously not set this flag by default, but only if
the user explicitly requested this behavior (and presumably knows how
to turn it back off if it fails).

Signed-off-by: Anton Khirnov <anton@khirnov.net>