Fbo mipmap fix

See: https://forum.libcinder.org/#Topic/23286000002113029

Mipmaps were not created for the FBO's textures. This fix moves the code that flags the FBO to update itselfs from the bindFrameBuffer() method to the unbindFrameBuffer() method. The latter must be made non-static for it to have access to the mObj member.

Please confirm that this will not break existing code before attempting to merge the code.

-Paul