Added rate-targeted temporal scalability

Added the ability to create rate-targeted, temporally
scalable, VP8 compatible bitstreams.

The application vp8_scalable_patterns.c demonstrates how
to use this capability. Users can create output bitstreams
containing upto 5 temporally separable streams encoded
as a single VP8 bitstream.
(previously abandoned as:
I92d1483e887adb274d07ce9e567e4d0314881b0a)

Change-Id: I156250a3fe930be57c069d508c41b6a7a4ea8d6a