Don't force batch flush when drawing group children

It's possible to define a custom `Batch` whose transform matrix may be changed without flushing. For subsequent draws, the vertices are simply adjusted in software. I expect the performance gain from longer batches to easily offset this transformation overhead (especially for 2D).

For my [prototype](http://pastebin.com/hDuggVwu) I had to do a nasty hack, because `Group` insists on flushing the batch when `transform` is set. Could we remove the `flush()` from `Group.drawChildren()`?