This patch adds the ability to snap strips to markers. Previously, there
only existed options to snap to hold offsets and the current frame.
This snap type works identically to other snapping options by checking
for the relevant bit (here `SEQ_SNAP_TO_MARKERS`) and adding the marker
frame numbers to `snap_data->target_snap_points` within
`seq_snap_target_points_build()`.
To enable `seq_get_snap_target_points_count()` to have access to marker
information, the current Scene object is now passed to the function.
Pull Request: https://projects.blender.org/blender/blender/pulls/120450
cff532e134