Adding syphon support to impress to allow access to the rendered output from other applications

[Tor Lillqvist]

> /slideshow/source/engine/OGLTrans/mac/OGLTrans_Shaders.m
>
> which has a function "CopyFramebufferToTexture" which I'm guessing copies
> the slideshow output to a texture?

That is the OpenGL slide *transition* code, not related to display of
slideshow in general.

(And it is unclear to me at least whether we even use it on the Mac,
despite there being a "mac" folder there in the source.)

--tml