RaP: When generating poll image, grab the presentation name correctly.

It was previously trying to get the presentation name from the wrong variable,
which resulted in a nil value (which was treated as a blank string in
filenames). This caused the poll images to not be inside the presentation
subdirectories, and the poll image references in the svg contained a `//`
path, which would break the recording if it was uploaded to e.g. AWS S3.

record-and-playback/presentation/scripts/publish/presentation.rb

@@ -368,7 +368,7 @@ def svg_render_shape_poll(g, slide, shape)
 
   result = JSON.load(shape[:result])
   num_responders = shape[:num_responders]
-  presentation = shape[:presentation]
+  presentation = slide[:presentation]
   max_num_votes = result.map{ |r| r['num_votes'] }.max
 
   dat_file = "#{$process_dir}/poll_result#{poll_id}.dat"