cocoa: remove dead code
cocoa: make resizing wake the vo thread
cocoa: make fullscreening look like an atomic operation

At the moment when you fullscreen mpv there is a very small time interval
where the fullscreen windows is semi-transparent. Apparently whem moving the
view to another parent it's better to make Cocoa not draw anything globally.

An Apple engineer said it, so it must be correct:
http://www.cocoabuilder.com/archive/cocoa/142020-preventing-flicker-on-moving-nsview-to-different-superview.html

I know I will regret this in the future.
cocoa: remove pointless drawRect

Apparently it causes deadlocks, and at the moment it does nothing.

Fixes #778