build: fix examples target paths

I 'broke' it in 78c362b. Sorry!
cocoa: move to a simpler threading model

Unfortunately using dispatch_sync for synchronization turned out to be really
bad for us. It caused a wide array of race conditions, deadlocks, etc.

Moving to a very simple mutex. It's not clear to me how to do liveresizing
with this, for now it just flickers with is unacceptable (maybe I'll draw
black instead).

This should fix all the threading cocoa bugs. Reopen if it's not the case!

Fixes #751
Fixes #1129