add bind function to avoid texture copy

[MartinHN]

Hi guys,
first thanks for your awesome work!

I've recently discovered that syphon copy texture if used via publish texture,
osx openGL profiler (and the code comments) says a second texture is used internally in syphon servers.

So I've tried to add a true bind functionality to draw directly in embedded syphon FBO using syphon's bindToDrawFrameOfSize method to avoid texture copy from openframeworks to syphonn.
few comments :

• I'm not an objective-C coder, so the few bits I've added there can be wrongly implemented
• I presume that having one texture instead of two is best but I'm not completly aware of syphon inner-mechanisms (texture allocation cost,safety of accessing texture from servers, multi threading ....)

if the above sounds right i think it can be seen as a possibility to reduce graphics memory by two for ofxSyphon projects isn't it?
furthermore using textureID from syphon we can have the texture back in oF for further use (using syphon as a simple ofFbo)

[EDIT]
the typical use case is :
oF generates a big texture for a client and still want to display it

let me know what you think

[bangnoise]

This is a leak - newFrameImage returns the image retained, and it needs to live for the lifetime of mTex (or until the next call to updateCurrentTexture()) but then be released

[bangnoise]

avoid formatting-only changes please (also further down)