Add FBO support for GLES

To support FBO for iOS (#160). Both the original framebuffer and renderbuffer must be restored after calling `nvgluCreateFramebuffer()` and the original framebuffer must also be restored when calling `nvgluBindFramebuffer(NULL)`. Because there are two functions need to reference the original framebuffer,  I added a global variable `default_fbo` to keep it, maybe there's a better way for that purpose. The code has been tested on iOS 7 and works fine.
No need to restore original renderbuffer here.