Fix for full-sample anti-aliasing (FSAA) not working with Freestyle.

Reverted the trunk revision 56136 and part of revision 56127 concerning the local Main
for Freestyle and temporary scene generation for stroke rendering.

The function do_merge_fullsample() in pipeline.c has access to the Scene of each Render,
so that the temporary Scene generated by Freestyle has to be kept for FSAA even after
stroke rendering has been done.  By the same token, the local Main has been moved from
the BlenderStrokeRenderer class to Render.  It is noted that free_all_freestyle_renders()
in pipeline.c is intended to get the temporary Scene of each Render released specifically
taking account of the FSAA case.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56253 954f8c5b-7b00-dc11-b283-0030488c597c