Fix #35561: freestyle + read full sample layers = crash.

Now add_freestyle() in pipeline.c takes a second argument to enable/disable
stroke rendering.  When stroke rendering is disabled, the function allocates
data structures but does not perform stroke rendering.  The allocated data
structures (mostly left unpopulated with data elements) are intended to allow
for the Read Full Sample Layers (Shift-R) command in the compositor.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57369 954f8c5b-7b00-dc11-b283-0030488c597c