vpx_ios/VPXExample: Fixes for iOS 8.

- plist HACK: Tell Xcode that Main_iPhone.storyboard is the launch screen.
  This works around a bug that appears to occur only when targeting the iPhone
  6 plus: For reasons unknown the simulator (and very likely the device based
  on observed behavior with older devices) reports incorrect dimensions when the
  application has no launch screen.
- OpenGL renderer:
    - compile error fix: The definitions of GL_RG_EXT and GL_RED_EXT
      are not longer supplied for free via the inclusion of GLKit/GLKit.h.
      Added include of glext.h.
    - orientation does influence device dimensions beginning with iOS 8
      (observed in simulator and on device). While changing this I've
      also taken the liberty of omitting the change of contentScaleFactor
      to 1.0, and using the device screen scale directly when setting up the
      view port.

Change-Id: If6949d9e4ef663f8aaa292b44a80abed552bd856