Import aom library

This is the reference implementation for the Alliance for Open Media's av1 video code.

The commit used was 4d668d7feb1f8abd809d1bca0418570a7f142a36.

third_party/aom/usage_cx.dox

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+/*! \page usage_encode Encoding
+
+    The aom_codec_encode() function is at the core of the encode loop. It
+    processes raw images passed by the application, producing packets of
+    compressed data. The <code>deadline</code> parameter controls the amount
+    of time in microseconds the encoder should spend working on the frame. For
+    more information on the <code>deadline</code> parameter, see
+    \ref usage_deadline.
+
+
+    \ref samples
+
+*/