vpx_ios: Fix playback on the iphone 6 plus.

The iphone 6 plus reports dimensions of 2208x1242 while openGL
rendering actually uses a 1920x1080 surface. The discrepancy
results in a cropped video image. Override the reported
dimensions when 2208x1242 is observed and use 1920x1080 instead.

Note: override of dimensions is disabled in the simulator; the sim
reports 2208x1242 and really is using a 2208x1242 surface.

Change-Id: I183de22ee7aeb377597be8d114ddae6398ec4458