Render some pixels in the window example.

This assumes a lot about padding and is very inefficient.

example/window.dart

@@ -1,12 +1,58 @@
 import 'package:x11/x11.dart';
 
+void drawWindow(X11Client client, X11ResourceId window, X11Rectangle area) {
+  //client.clearArea(window, area);
+  var gc = client.generateId();
+  client.createGC(gc, window);
+
+  // Write image in tiles (can't fit full image data into a single call).
+  var tile_size = 100;
+  for (var y0 = 0; y0 < area.height; y0 += tile_size) {
+    var y1 = y0 + tile_size;
+    if (y1 > area.height) {
+      y1 = area.height;
+    }
+
+    for (var x0 = 0; x0 < area.width; x0 += tile_size) {
+      var x1 = x0 + tile_size;
+      if (x1 > area.width) {
+        x1 = area.width;
+      }
+
+      var tile_area = X11Rectangle(
+          x: area.x + x0, y: area.y + y0, width: x1 - x0, height: y1 - y0);
+
+      // FIXME: Padding should be taken from X11Format
+      var data = <int>[];
+      for (var y = 0; y < tile_area.height; y++) {
+        for (var x = 0; x < tile_area.width; x++) {
+          data.add(255 * (tile_area.x + x) ~/ area.width);
+          data.add(255 * (tile_area.y + y) ~/ area.height);
+          data.add(255);
+          data.add(0);
+        }
+
+        // Each row needs to be padded.
+        while (data.length % 4 != 0) {
+          data.add(0);
+        }
+      }
+      client.putImage(gc, window, tile_area, data);
+    }
+  }
+  client.freeGC(gc);
+}
+
 void main() async {
   var client = X11Client();
   await client.connect();
 
   var wmProtocolsAtom = await client.internAtom('WM_PROTOCOLS');
   var wmDeleteWindowAtom = await client.internAtom('WM_DELETE_WINDOW');
 
+  client.errorStream.listen((error) async {
+    print('$error ${error.majorOpcode}.${error.minorOpcode}');
+  });
   client.eventStream.listen((event) async {
     if (event is X11KeyPressEvent) {
       print('KeyPress ${event.key}');
@@ -27,7 +73,7 @@ void main() async {
     } else if (event is X11FocusOutEvent) {
       print('FocusOut');
     } else if (event is X11ExposeEvent) {
-      client.clearArea(event.window, event.area);
+      drawWindow(client, event.window, event.area);
     } else if (event is X11ClientMessageEvent) {
       if (event.type == wmProtocolsAtom) {
         if (X11Atom(event.data[0]) == wmDeleteWindowAtom) {