added ability to publish a gl::Texture for current FBO support

[nselikoff]

Overloading publishTexture so that one can do:

mSyphon.publishTexture( mFbo.getTexture() );

Not my idea, but I need it for a project; see https://forum.libcinder.org/topic/an-easier-cinder-syphon for discussion.

[gaborpapp]

thanks for this. without this functionality it is quite cumbersome to publish an Fbo texture.