Changes

OSX Fixes: bug 548 (another regression: pixelfmt), ctx creation failure -> no exception,

- bug 548: Another regression: pixelfmt failed for 10.6.7 and/or software OpenGL
  - enforcing accelerated leads to no pixelformat,
  - using the NSOpenGLView defaultPixelFormat causes to SIGSEGV

- ctx creation failure shall just lead to return null, no immediate exception

OSX Fix: Catch releaseImpl's glFinish() exception (DebugGL); Make GLContext.release's setCurrent(null) exception prone.

Catch releaseImpl's glFinish() exception (DebugGL)
  glGetError() after glFinish() (eg. w/ debug pipeline) produced unknown error 0x0506
  on OS X (10.7.2 NV).

Make GLContext.release's setCurrent(null) exception prone
  Call setCurrent(null) in finalizer block to ensure it's 'released' out of the TLS
  even when an exception is being thrown.

Make MacOSX Shared Resources 'destroy' more error prone (catch exceptions)

NEWT/OSX Performance Fix: Cache CGDisplayScreenSize() result, since it's ridiculous slow

Each call to CGDisplayScreenSize() took around 6ms (5ms .. 20ms, avrg 6ms)
which added up to ~2s for ~400 Screen modes.