Lines loose extraction cannot happen on the fly as the render data isn't
complete. This patch will do the lines loose subbuffer extraction as a
simple task and not on the fly.

This patch introduces 2 sub-types of tasks that are executed single threaded.
EXTRACT_MESH_EXTRACT would do the extraction using Mesh/BMesh geometry.
EXTRACT_LINES_LOOSE creates the lines_loose subbuffer from already cached
lines IBO.