present: add background image element for slides

Fixes golang/go#11470

Change-Id: I665622197bea48b95adc839be8de7d03bf812f37
Reviewed-on: https://go-review.googlesource.com/18360
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/present/static/styles.css

@@ -384,6 +384,20 @@ article > .image {
   margin-top: 40px;
 }
 
+article > .background {
+  position: absolute;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  z-index: -1;
+}
+
+article > .background > img {
+  max-height: 100%;
+  max-width: 100%;
+}
+
 table {
   width: 100%;
   border-collapse: collapse;

cmd/present/templates/action.tmpl

@@ -37,6 +37,12 @@ It determines how the formatting actions are rendered.
 </div>
 {{end}}
 
+{{define "background"}}
+<div class="background">
+  <img src="{{.URL}}">
+</div>
+{{end}}
+
 {{define "iframe"}}
 <iframe src="{{.URL}}"{{with .Height}} height="{{.}}"{{end}}{{with .Width}} width="{{.}}"{{end}}></iframe>
 {{end}}

present/background.go

@@ -0,0 +1,25 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package present
+
+import (
+	"strings"
+)
+
+func init() {
+	Register("background", parseBackground)
+}
+
+type Background struct {
+	URL string
+}
+
+func (i Background) TemplateName() string { return "background" }
+
+func parseBackground(ctx *Context, fileName string, lineno int, text string) (Elem, error) {
+	args := strings.Fields(text)
+	background := Background{URL: args[1]}
+	return background, nil
+}

present/doc.go

@@ -58,6 +58,7 @@ After that come slides/sections, each after a blank line:
 	.code x.go /^func main/,/^}/
 	.play y.go
 	.image image.jpg
+	.background image.jpg
 	.iframe http://foo
 	.link http://foo label
 	.html file.html
@@ -177,6 +178,12 @@ preserves the aspect ratio of the image when scaling.
 
 	.image images/janet.jpg _ 300
 
+background:
+
+The template uses the function "background" to set the background image for
+a slide.  The only argument is the file name of the image.
+
+	.background images/susan.jpg
 
 caption: