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.
Reviewed By: Clément Foucault
Differential Revision: https://developer.blender.org/D7964
624994caa2