Made an attempt for fixing an instability issue regarding animation

rendering.

RE_BlenderFrame() and RE_BlenderAnim() are Blender's internal
top-level rendering API functions for a single frame and a series of
frames, respectively.  These functions rely on global variables to
keep render pipeline states.  Freestyle's stroke rendering was also
using RE_BlenderFrame to render a temporary scene of stroke meshes.
These nested calls of the top-level rendering API functions were
causing broken render pipeline states and eventually led to a crash.

This commit is intended to fix the issue, by introducing an rendering
API function RE_RenderFreestyleStrokes specifically used for stroke
rendering in Freestyle.  The new function does not do anything with
regard to the global variables in the rendering API implementation.

source/blender/render/extern/include/RE_pipeline.h

@@ -216,6 +216,7 @@ void RE_TileProcessor(struct Render *re, int firsttile, int threaded);
 /* only RE_NewRender() needed, main Blender render calls */
 void RE_BlenderFrame(struct Render *re, struct Scene *scene, struct SceneRenderLayer *srl, int frame);
 void RE_BlenderAnim(struct Render *re, struct Scene *scene, int sfra, int efra, int tfra, struct ReportList *reports);
+void RE_RenderFreestyleStrokes(struct Render *re, struct Scene *scene);
 
 void RE_ReadRenderResult(struct Scene *scene, struct Scene *scenode);
 void RE_WriteRenderResult(RenderResult *rr, char *filename, int compress);