vf_lavfi: recreate filter graph on seek

Resetting the filter graph helps dealing with filters which save state
between frames. This is important especially if they modify frame timing
or emit additional frames.

Unfortunately the libavfilter API doesn't have a way to do this
directly, so we have to use a dirty trick: we recreate the whole graph,
including format negotiation down and filter string parsing. ffplay does
this too. If libavfilter somehow decides to change output format or size
from what the first run in config() returned, mpv will explode. The same
applies to vf_next_query_format() return values (although this could be
mitigated, should it really happen).
core: don't let cache pause handling and user pausing conflict

The core pauses and unpauses automatically to wait for the network
cache (also known as buffering). This conflicted with user pause
control, and was perceived as if the player was unresponsive and/or
the cache just overturned the user's decisions.

Change it so that the actual pause state and the pause state as
intended by the user never conflict. If the user toggles pause, the
pause state will be in the expected state as soon as the cache is
loaded.
core: simplify handling of --pause

Rename the struct MPOpts "start_pause" field to "pause". Store the user-
pause state in that field, so that both runtime pause toggling and the
--pause switch change the same variable. Simplify the initialization of
pause so that using --pause and changing the file while paused is
exactly the same case (changing the file while paused doesn't unpause,
this has been always this way).

Also make it a bit more consistent. Before, starting with --pause would
reset the pause state for every file, instead of following the usual
semantics for option switches (compare with behavior of --fs).