x/tools/present: add video element for slides

Change-Id: I19f7c181d9bd8148f7791925f04207a34c95ac1e
Reviewed-on: https://go-review.googlesource.com/18484
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/present/static/styles.css

@@ -379,7 +379,8 @@ code {
   color: black;
 }
 
-article > .image {
+article > .image,
+article > .video {
   text-align: center;
   margin-top: 40px;
 }

cmd/present/templates/action.tmpl

@@ -37,6 +37,14 @@ It determines how the formatting actions are rendered.
 </div>
 {{end}}
 
+{{define "video"}}
+<div class="video">
+  <video {{with .Height}} height="{{.}}"{{end}}{{with .Width}} width="{{.}}"{{end}} controls>
+    <source src="{{.URL}}" type="{{.SourceType}}">
+  </video>
+</div>
+{{end}}
+
 {{define "background"}}
 <div class="background">
   <img src="{{.URL}}">

present/doc.go

@@ -178,6 +178,22 @@ preserves the aspect ratio of the image when scaling.
 
 	.image images/janet.jpg _ 300
 
+video:
+
+The template uses the function "video" to inject video files.
+
+The syntax is simple: 2 or 4 space-separated arguments.
+The first argument is always the file name.
+The second argument is always the file content-type.
+If there are more arguments, they are the height and width;
+both must be present, or substituted with an underscore.
+Replacing a dimension argument with the underscore parameter
+preserves the aspect ratio of the video when scaling.
+
+	.video videos/evangeline.mp4 video/mp4 400 600
+
+	.video videos/mabel.ogg video/ogg 500 _
+
 background:
 
 The template uses the function "background" to set the background image for

present/video.go

@@ -0,0 +1,51 @@
+// 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 (
+	"fmt"
+	"strings"
+)
+
+func init() {
+	Register("video", parseVideo)
+}
+
+type Video struct {
+	URL        string
+	SourceType string
+	Width      int
+	Height     int
+}
+
+func (v Video) TemplateName() string { return "video" }
+
+func parseVideo(ctx *Context, fileName string, lineno int, text string) (Elem, error) {
+	args := strings.Fields(text)
+	vid := Video{URL: args[1], SourceType: args[2]}
+	a, err := parseArgs(fileName, lineno, args[3:])
+	if err != nil {
+		return nil, err
+	}
+	switch len(a) {
+	case 0:
+		// no size parameters
+	case 2:
+		// If a parameter is empty (underscore) or invalid
+		// leave the field set to zero. The "video" action
+		// template will then omit that vid tag attribute and
+		// the browser will calculate the value to preserve
+		// the aspect ratio.
+		if v, ok := a[0].(int); ok {
+			vid.Height = v
+		}
+		if v, ok := a[1].(int); ok {
+			vid.Width = v
+		}
+	default:
+		return nil, fmt.Errorf("incorrect video invocation: %q", text)
+	}
+	return vid, nil
+}