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.
Unfortunately this variable needs to be restored in both nvgluCreateFramebuffer() and nvgluBindFramebuffer() functions. Or in iOS, if you just call nvgluCreateFramebuffer() and do nothing. You won't be able to render anything as usual.