vpx_ios/VPXTest: Add video buffer pool.

Add VideoBufferPool with kNumVideoBuffers of 8. Instead of leaking
CVPixelBufferRefs to GlkVideoViewController, VpxPlayer instead uses
VideoBufferPool to manage buffers. Buffers are preallocated immediately
after container parsing initialization is completed.
VpxPlayer obtains buffers from the pool via VideoBufferPool::GetBuffer(),
and copies the output image to the buffer. It then hands the buffer off
to GlkVideoViewController for display via :receiveVideoBuffer. After
display, GlkVideoViewController returns the buffer to VpxPlayer via
VpxPlayer::ReleaseVideoBuffer(), which is a simple wrapper for
VideoBufferPool::ReleaseBuffer().

Change-Id: I0b2c7d5938e5bdeafdacd63d69df3e50bcc65129