Batch previews: improve speed of preview generation.

Force multi-threading of preview renders by setting appropriate tile
sizes (by default it would typically render on one or two threads only).

release/scripts/modules/bl_previews_utils/bl_previews_render.py

@@ -135,6 +135,9 @@ def do_previews(do_objects, do_collections, do_scenes, do_data_intern):
         scene.render.filepath = os.path.join(bpy.app.tempdir, 'TEMP_preview_render.png')
         scene.render.use_overwrite = True
         scene.render.use_stamp = False
+        scene.render.threads_mode = 'AUTO'
+        scene.render.tile_x = RENDER_PREVIEW_SIZE // 4
+        scene.render.tile_y = RENDER_PREVIEW_SIZE // 4
 
         image = bpy.data.images.new("TEMP_render_image", RENDER_PREVIEW_SIZE, RENDER_PREVIEW_SIZE, alpha=True)
         image.source = 'FILE'