We need to rebind the framebuffer prior to painting layers.

Otherwise a surface drawing call in paint() will bind that surface's frame
buffer and we'll render our layers there instead of to the default frame
buffer. Obviously not desired.

It turns out that IOSGLContext was already doing the right thing.

core/src/playn/core/gl/GL20Context.java

@@ -65,6 +65,7 @@ public void preparePaint(GroupLayerGL rootLayer) {
 
   public void paintLayers(GroupLayerGL rootLayer) {
     checkGLError("paintLayers");
+    bindFramebuffer();
     rootLayer.paint(rootXform, 1, null); // paint all the layers
     useShader(null, false); // flush any pending shader
   }