Bugfix: really use closest enabled marker for track initialisation

Previously we relied on BKE_tracking_marker_get, which contains magic to
fall back on the "closest" (actually next to left) frame, in case there
isn't an exact match.

Unfortunately, when refining tracks interactively, Blender has the habit
to set an additional marker to the previous frame with flag == disabled,
in order to mark the start of an active track segment. Thus, if we don't
observe the enabled state of a marker, we might end up using some arbitrary
incorrect tracking values, which just happen to sit in such a toggle marker.

When this happens during initialisation, these arbitrary values can poison
every other measurement value on that track and any other tracks chained behind.

The fix here is to re-use the search functions implemented for interpolation
of full gaps. This gives us the nice additional benefit of really picking
the marker closest to the the desired reference frame
Workaround: disable track weights for initialisation

this workaround allows to use track weights in practice, while the proper
access to keyframed values is not yet implemented.

Explanation: for initialisation, we need to pick the measured values at
various reference frames, which are obviously not at the global "current frame"
position and thus don't hold the correct keyframed values. Typically this leads
to baseline values not being picked up at all (when the respective tracks
have been faded out through animation)