Add support for touch events to sky_viewer

sky/examples/touch-demo.sky

Index: sky/examples/touch-demo.sky
diff --git a/sky/examples/touch-demo.sky b/sky/examples/touch-demo.sky
new file mode 100644
index 0000000000000000000000000000000000000000..26c659bc2a5ddd9401ac116d76efbbb2c440668c
--- /dev/null
+++ b/sky/examples/touch-demo.sky
@@ -0,0 +1,16 @@
+<sky>
+<div id="log">Ready</div>
+<script>
+function logTouchEvent(evt) {
+  var message = "type=" + event.type;
+  if (evt.touches && evt.touches.length > 0)
+    message += " x=" + evt.touches[0].clientX + " y=" + evt.touches[0].clientY;
+  document.getElementById("log").textContent = message;
+}
+
+document.documentElement.addEventListener("touchstart", logTouchEvent);
+document.documentElement.addEventListener("touchmove", logTouchEvent);
+document.documentElement.addEventListener("touchend", logTouchEvent);
+document.documentElement.addEventListener("touchcancel", logTouchEvent);
+</script>
+</sky>