gosrc: handle ".background" in presentations

The current implementation does not resolve background images, therefore
they do not display on talksapp. This change handles .background in the
same way as .image and .iframe.

Fixes golang/gddo#541

Change-Id: I309688fb95bc407fb59e8eb4c5e5ea2f3c54b184
Reviewed-on: https://go-review.googlesource.com/c/133235
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
8 files changed