Sometimes a server needs to change its context without disconnecting clients

[bangnoise]

http://v002.info/forums/topic/client-loses-connection-when-server-changes-context/#post-29202

http://v002.info/forums/topic/syphoninject/#post-27198

[bangnoise]

SyphonServer will probably gain

@property (readwrite) CGLContextObj context;

[vade]

Strategy here is to keep the IOSurface around and rebuild the GL context / resources around it, if we need to. Similar to how resize is handled (which requires a new IOSurface too)