Fixes issue #543 for mChildren in RajawaliRender. For loops which
iterate over mChildren now check the length on each cycle through the
loop. I did not include this for mPlugins because I can see no reason
why this would be changing at a frame by frame level. 

Signed-off-by: Jared Woolston <jwoolston@tenkiv.com>
Merge pull request #546 from Tenkiv/concurrency_renderer

Fixes issue #543 for mChildren in RajawaliRender.