add scalability experiment

configure:  --enable-experimental --enable-scalability

New applications:  scalable_encoder, scalable_decoder

scalable_encoder:
  * Encodes inputs as 2-layer (same size) stream
  * Encodes as obu file (OBU_NO_IVF must be enabled)
  * Base layer encoded in IPPPP where P's reference
    only the previous (in time) base layer
  * Enhancement layer encoded using its base layer as
    sole reference frame
  * Base layer encoded with fixed high QP
  * Enhancement layer encoded with fixed low QP

scalable_decoder:
  * Able to decode scalable stream generated by
    scalable_encoder
  * Able to decode any single-layer stream encoded
    by aomenc
  * Outputs base layer as out_lyr0.yuv, and enhancement
    layer (if they exist) as out_lyrN.yuv (N = 1, 2, 3, ..)
  * Able to decode N layers (more than 2)

Change-Id: I8555735db71e5b9b6f900ffdf978e0ad6f6bfc00