Add a method to use a new shared EAGLContext

I pulled the code from http://forum.sparrow-framework.org/topic/thread-safety-when-rendering-spimages#post-9344 into useNewSharedEAGLContext on SPView. It creates a new EAGLContext with the main context's sharegroup. It then makes it the context for the current thread.

This is part of issue #1019.