Freestyle SVG export: handle relative paths

Previously, relative paths were not handled well. The result was an
error in the console and no output at the expected location.

The path resolving order is as follows:

* use the given path (scene.render.frame_path()) when it is absolute
* otherwise, try to join with the current blendfile's location
* otherwise, try to join with the folder from which blender was called

Tested on linux, but not on Windows

render_freestyle_svg.py

@@ -151,12 +151,28 @@ def is_preview_render(scene):
 
 def create_path(scene):
     """Creates the output path for the svg file"""
-    dirname = os.path.dirname(scene.render.frame_path())
+    path = os.path.dirname(scene.render.frame_path())
+    file_dir_path = os.path.dirname(bpy.data.filepath)
+
+    # try to use the given path if it is absolute
+    if os.path.isabs(path):
+        dirname = path
+
+    # otherwise, use current file's location as a start for the relative path
+    elif file_dir_path:
+        dirname = os.path.normpath(os.path.join(file_dir_path, path))
+
+    # otherwise, use the folder from which blender was called as the start
+    else:
+        dirname = os.path.abspath(bpy.path.abspath(path))
+
+
     basename = bpy.path.basename(scene.render.filepath)
     if scene.svg_export.mode == 'FRAME':
         frame = "{:04d}".format(scene.frame_current)
     else:
         frame = "{:04d}-{:04d}".format(scene.frame_start, scene.frame_end)
+
     return os.path.join(dirname, basename + frame + ".svg")
 
 
@@ -741,4 +757,3 @@ def unregister():
 
 if __name__ == "__main__":
     register()
-