AVFoundation (OS X): use AVPlayerItemVideoOutput for rendering frames

There are currently two frame rendering paths in the AVFoundation plugin, one for OS X and one for iOS. On OS X, we render an AVPlayerLayer into an FBO. On iOS, we use AVPlayerItemVideoOutput to retrieve pixel buffers, which can then be mapped to memory or to an OpenGL texture.

We should be using AVPlayerItemVideoOutput on OS X as well, since the API is available there too. The only problem is that OpenGL textures created from a pixel buffer are of type GL_TEXTURE_RECTANGLE on OS X, which is not currently supported by the QML video node.